Sometimes new belts can squeak at first until they bed in, likewise if pulleys freshly painted or powdercoated until the surface wears. But generally a squeak or squeal is from belt slipping just enough to move. Nip them up a tad tighter, inspect the belts and pulleys to see any new wear or patterns that didn't match previous. Under load will make it worse, turning AC on, power steering at close to full-lock, revving the engine, anything like that will squeal more when done. If just contact make sure the belt size and type is correct, not out of alignment or pulley order.