2020-B4-2

Author Topic: Whistling sound on deceleration and in gear...  (Read 313 times)

nUcLeArEnVoY

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Whistling sound on deceleration and in gear...
« on: July 31, 2020, 06:15:48 AM »
Happened twice now; both times at night and after car is well warmed up.

- Makes a shrill, consistent whistling sound
- Only happens when car is in motion and I let off the throttle while in gear and let it coast. If I depress the clutch pedal and/or go into neutral while coasting, it quiets down and stops. It also stops if I open the throttle and give it gas. When it does quiet down, the sound "fades" out over a second rather than stops abruptly
- Happens in all gears and at any speed; noticeably louder at lower speeds/lower gears
- Doesn't happen when idling or even revving the motor at idle when the RPMs drop.

First time it happened was a few weeks ago and I was leaning towards transmission needing more lubricant since I have leaks (which still aren't fixed), so I changed the fluid and topped it off. Took it for a drive a few days ago and no sound so I thought it was fixed and that it was the tranny needing lube, but then it happened again last night.

Now I'm considering vacuum leak somewhere since it happens on deceleration when vac is highest, but if that were the case, then it should also occur at idle, and should occur regardless of whether it's in and out of gear. I've been allover the entire engine with propane, brake cleaner, water, even a homemade smoke machine and can't find any leaks (besides smoke exiting the top of the carb which is expected). Only place I haven't checked for leaks are the AC/Dash controls. Car idles smooth (a little lopey but I think it has a mild cam) between 775-825 RPM depending on how warmed up it is, and pulls 16" of manifold vac.

It doesn't SOUND like it's coming from the transmission, but I know that don't mean much.

Car runs fine otherwise. Any help would be appreciated.
« Last Edit: July 31, 2020, 08:30:45 AM by nUcLeArEnVoY »
1979 Trans Am 400/4-Speed W72/WS6 - Starlight Black Hardtop

firebirdparts

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Re: Whistling sound on deceleration and in gear...
« Reply #1 on: July 31, 2020, 06:57:38 AM »
There's more vacuum when you are decelerating than there is idling.  Something like the carb  base gasket could be acting like a reed.  It's a strange problem though.
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1977 Y82 4-speed (red interior)
1978 Skybird 350
1978 Redbird 305
1979 Redbird 301 4-speed
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b_hill_86

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Re: Whistling sound on deceleration and in gear...
« Reply #2 on: July 31, 2020, 06:58:00 AM »
“ Now I'm considering vacuum leak somewhere since it happens on deceleration when vac is highest, but if that were the case, then it should also occur at idle, and should occur regardless of whether it's in and out of gear.“

Don’t have an answer for you but just remember manifold vacuum is a lot higher on deceleration than it is on idle and will only be higher on deceleration than idle when in gear. Again, not a definitive answer. It just food for thought.
-Brian-

1977 Trans Am 400 4 speed

b_hill_86

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Re: Whistling sound on deceleration and in gear...
« Reply #3 on: July 31, 2020, 07:00:39 AM »
Beat me by less than 30 seconds lol
-Brian-

1977 Trans Am 400 4 speed

nUcLeArEnVoY

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Re: Whistling sound on deceleration and in gear...
« Reply #4 on: July 31, 2020, 07:16:26 AM »
“ Now I'm considering vacuum leak somewhere since it happens on deceleration when vac is highest, but if that were the case, then it should also occur at idle, and should occur regardless of whether it's in and out of gear.“

Don’t have an answer for you but just remember manifold vacuum is a lot higher on deceleration than it is on idle and will only be higher on deceleration than idle when in gear. Again, not a definitive answer. It just food for thought.

Can you explain why it's higher in gear than in neutral, even while in motion and coasting? So that I can understand.

Also, Joe, funny you mention carb gasket. FWIW I did replace the carb base gasket recently. Pretty sure before the first time this happened, too. Replaced it with a Felpro from rock auto. Mounting bolts are torqued to 12 ft-lbs as per 1978 service manual (it specifically states 145 inch-lbs, so I converted and rounded down). Should I tighten them a bit and see if that changes anything? I don't want to overtighten and risk an airhorn warp, if there isn't already one. I also never re-tightened them to account for new gasket setting/compression from heat/use, so I guess I should at least do that...

 Would a manifold vacuum leak produce such a racket?

Car is a 1979 W72, btw. Original everything other than maybe this potential camshaft.
« Last Edit: July 31, 2020, 07:58:57 AM by nUcLeArEnVoY »
1979 Trans Am 400/4-Speed W72/WS6 - Starlight Black Hardtop

81Blackbird

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Re: Whistling sound on deceleration and in gear...
« Reply #5 on: July 31, 2020, 08:20:17 AM »
When you said cicadas I immediately thought of a loose belt.  Wouldn't hurt to check. 

nUcLeArEnVoY

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Re: Whistling sound on deceleration and in gear...
« Reply #6 on: July 31, 2020, 08:31:30 AM »
When you said cicadas I immediately thought of a loose belt.  Wouldn't hurt to check.

Dont think its belts. Maybe I used a bad comparison, the sound doesn't fluctuate at all it's a constant and consistent whistle.
1979 Trans Am 400/4-Speed W72/WS6 - Starlight Black Hardtop

b_hill_86

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Re: Whistling sound on deceleration and in gear...
« Reply #7 on: July 31, 2020, 08:59:13 AM »
Think about it this way. When you’re idling, your engine “wants” to increase RPM but the air being sucked in by the intake strokes on each cylinder is restricted by closed throttle blades. This creates the vacuum at idle because the cylinders are trying to pull in more air than what’s available at a given RPM. For example, when I was younger I replaced an exhaust manifold on my 4.0L I-6 in my Jeep. Since the exhaust manifold and intake manifold share some bolts on that engine I removed the intake too. When I replaced it I didn’t have the intake set on the dowel pins that were present so there was a gap between the intake and head.  When I started it it revved uncontrollably as if I had the accelerator fully pressed because it was able to suck in all the air it wanted.

Anyway, when you’re in gear and coasting with the clutch out your engine is obviously connected to the rear wheels via the drivetrain so the faster you’re rolling the higher your RPM is even though your throttle is closed so your engine is trying to continue to move air into the cylinders (a lot more than when idling) but can’t because your throttle blades are closed. This creates increased vacuum.

I’m not an expert or engineer or anything so I hope I made sense explaining that.
« Last Edit: July 31, 2020, 06:25:50 PM by b_hill_86 »
-Brian-

1977 Trans Am 400 4 speed

81Blackbird

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Re: Whistling sound on deceleration and in gear...
« Reply #8 on: July 31, 2020, 09:09:01 AM »
b hill said it nicely.  High RPM with the throttle close the engine is looking for air.  There is a little in the brake booster and vacuum can...or a gasket leak or vacuum connection.  Try spraying some carb cleaner or break cleaner around those areas.  If the RPM increased you may have found your cicada.
« Last Edit: July 31, 2020, 10:42:26 AM by 81Blackbird »

b_hill_86

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Re: Whistling sound on deceleration and in gear...
« Reply #9 on: July 31, 2020, 06:08:57 PM »
Still not an answer to your noise but I took my car to get dinner tonight so I took a video of my vacuum gauge just to illustrate vacuum on deceleration. Idle for me is 17-18  inches or so of mercury at idle.

https://youtu.be/yjJCDmHSq8M
-Brian-

1977 Trans Am 400 4 speed

nUcLeArEnVoY

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Re: Whistling sound on deceleration and in gear...
« Reply #10 on: July 31, 2020, 06:14:41 PM »
Still not an answer to your noise but I took my car to get dinner tonight so I took a video of my vacuum gauge just to illustrate vacuum on deceleration. Idle for me is 17-18  inches or so of mercury at idle.

https://youtu.be/yjJCDmHSq8M

I see. Maybe it needs to be a very high vacuum signal to make the noise.
1979 Trans Am 400/4-Speed W72/WS6 - Starlight Black Hardtop

Jack

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Re: Whistling sound on deceleration and in gear...
« Reply #11 on: July 31, 2020, 06:36:36 PM »
I've had the same problem for years, comes and goes and I don't think it's the vacuum.




Regards, Jack

nUcLeArEnVoY

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Re: Whistling sound on deceleration and in gear...
« Reply #12 on: August 01, 2020, 10:41:17 AM »
I've had the same problem for years, comes and goes and I don't think it's the vacuum.

Yeah, it seems to come and go for me, too but then it's only happened twice. Coincidentally both times after I put on the new carb base gasket so I'll definitely retighten those bolts.

What do you suspect it may be? Have you looked into it?
1979 Trans Am 400/4-Speed W72/WS6 - Starlight Black Hardtop

Fbird

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Re: Whistling sound on deceleration and in gear...
« Reply #13 on: August 02, 2020, 05:43:19 AM »
remove your breather ...see if it gets louder....could actually be the carb ...getting air sucked thru it past the throttle plates (don't forget to block off any thermal switch vacuum lines runing to the breather assembly)
67 Firebird
1981 Turbo TA (injected 400 now :) )

nUcLeArEnVoY

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Re: Whistling sound on deceleration and in gear...
« Reply #14 on: August 02, 2020, 06:01:26 AM »
Think I may have found the issue, guys.

Haven't taken it on a drive yet, but I just realized yesterday that the torque wrench I used to tighten the carb mounting bolts after I put on the new base gasket is calibrated in inch-pounds, not ft-lbs. When I originally installed the gasket, I thought it was ft-lbs and had it set to 12 (144 in-lbs)... which means I actually had it set to 1 ft-lb since the wrench is calibrated in inch lbs (it's a 1/4" torque wrench). That would explain why I never felt an audible click, lol. I probably well exceeded 12 in-lbs within the first turn, and ultimately didn't tighten them to the proper specs (145 in-lbs for the front long bolts and 168 in-lbs for the rear short bolts), so the mounting bolts were very much undertightened. I tightened them to the correct spec and actually felt a noticeable click this time, so when I get the new ignition coil and distributor cap/rotor on, I'll take it out on a drive. Hope that solves it. After all, there was no noise all the times I drove the car before I put on the new carb gasket

And Fbird, you mean remove my PCV breather on the valve cover? Not sure what you're talking about other than that, and it doesn't have vacuum lines running to it.
« Last Edit: August 02, 2020, 06:22:05 AM by nUcLeArEnVoY »
1979 Trans Am 400/4-Speed W72/WS6 - Starlight Black Hardtop

Re: Whistling sound on deceleration and in gear...
« Reply #14 on: August 02, 2020, 06:01:26 AM »


2020-B1-8


81Blackbird

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Re: Whistling sound on deceleration and in gear...
« Reply #15 on: August 02, 2020, 09:59:12 AM »
Spraying the carb/break cleaner may have helped.  Glad you found something that could cause the sound.

Fbird

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Re: Whistling sound on deceleration and in gear...
« Reply #16 on: August 03, 2020, 04:46:33 AM »
no i was refering to remove the entire BREATHER assembly....to see if it was whistling  THRU the carb.(would get louder without the breather) Some breather assemblies have a thermal vacuum switch associated with the fresh air opening.

Glad you solved your issue...surprised you were not getting a lean surge while cruising.
67 Firebird
1981 Turbo TA (injected 400 now :) )

nUcLeArEnVoY

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Re: Whistling sound on deceleration and in gear...
« Reply #17 on: August 03, 2020, 06:35:47 AM »
no i was refering to remove the entire BREATHER assembly....to see if it was whistling  THRU the carb.(would get louder without the breather) Some breather assemblies have a thermal vacuum switch associated with the fresh air opening.

Glad you solved your issue...surprised you were not getting a lean surge while cruising.

I actually didnt solve it. Retightened the bolts and still have the whistle. Wondering if I have an intake gasket leak...


When you say breather assembly, you mean the tube coming from the passenger valve cover to the air cleaner? I did recently replace the rubber grommet for it on the valve cover as the old one was super worn and caused the breather tube to have a lot of play. Also added a filter element. The grommet and tube are very snug now.
« Last Edit: August 03, 2020, 07:06:55 AM by nUcLeArEnVoY »
1979 Trans Am 400/4-Speed W72/WS6 - Starlight Black Hardtop

T/Atom

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Re: Whistling sound on deceleration and in gear...
« Reply #18 on: August 03, 2020, 09:59:53 AM »
My car makes a similar whistle to the one you describe, and I have realized it is/was electrical interference noise from the speakers. I only notice it under certain conditions, like when I’m off the throttle and can actually hear it over the exhaust/wind noise with the windows open etc...just a thought. It’s not a factory setup, I have an amp and crossovers with aftermarket speakers and tweeters. 

Re: Whistling sound on deceleration and in gear...
« Reply #18 on: August 03, 2020, 09:59:53 AM »
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