Squealing sound at idle only (91 Toyota Corolla

My manual transmission 91 Toyota Corolla has recently began to make a high pitched whining/squealing noise. This noise occurs only when the engine is idling (such as when stopped at a stoplight). If the engine is revved very slightly above idle, the noise disappears. It is rather loud, though until I drove with a window opened, I had not realized it was that loud.

This has only started within the last month. Soon before the noise began, I tightened one the belts to stop it from slipping and screaching in the cold weather whenever I would start the car. Could the tightened belt be causing the sound or is it something else?

Sometimes, you have to change a belt rather than tighten it. If the belt is already tight, and it’s squealing and slipping, a belt dressing (available at auto parts stores) can stop the slipping. If it still slips and squeals, replace it.

I should note that tightening the belt fixed the previous sound, and that this is a new type of squeal.

Oh, there’s any number of noises your belts can make! It’s almost certainly just an old belt, but while you’re in there changing them, you should give all the pulleys a few spins to make sure there isn’t something with a sticking bearing.

There is absolutely no reason to believe that a belt that is NOT slipping would align as you say.

This would require that the belt length be precisely a multiple of the perimeter of the marked pulley. There’s no reason to believe this is true. In short, this is not a valid test.

And belt dressing, including wax candles, hand soap, etc., are only temporary fixes. Be sure not to exceed the recommended belt tension. As previously stated, while you’re checking the belts, after at least relieving all of the tension on the belts if you don’t just remove them, fiddle around with all of the pulleys to check for looseness, wobbling, etc. It could be a worn bushing or bearing causing the belt squeal or it could be a worn belt. Belts are much less expensive to replace than bushings and/or bearings. And with serpentine belts, that serp. belt is a heck of a lot cheaper to replace than
say, an idler pulley. But ‘while you’re there’, check all of these things. If in any doubt about the belt(s), replace them.