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I've known for years that a belt squeal is usually due to a worn/old/misadjusted belt.
I currently have a 2007 Toyota Yaris that has an intermittent belt squeal on startup, I'm guessing it's just due to it being an old belt. Aside from being super annoying, is there anything harmful about waiting to replace the belt until it always squeals, or should I go ahead and replace it now?
It squeals on startup because the alternator is making the most power at that time. The battery has been lowered a bit by the engine start, and the alternator is refilling that. Since the battery was near full, the alternator quickly tapers off to a lower charge rate, and the squealing stops on its own.
However, if you run a lot of accessories (as you will on a rainy day when you have lights, wipers, rear defog and A/C all going to town)... the squealing will return and not let up.
Inspect the equipment. Look for oil contamination on the belt. Look for glazing on the pulleys. Look at belt condition. Fix the oil leak that is causing the contamination - cleaning oil off a belt is prohibitive and replace belt if necessary. Belts are cheap LOL. Wire-brush any glazing off the pulleys - the pulleys that matter are the alternator and crank pulley.
I would say the #1 cause of belt squeal is the belt not being tight enough. Tighten the belt properly - typically more than one might expect, and they don't make it easy - you often have to pry on the alternator in weird ways. A few automakers stick a 3/8" or 1/2" square hole somewhere useful so you can stick a breaker bar on it.
Lastly, don't ignore the possibility that the squeal is a failing bearing in the alternator. They take a tremendous amount of thrust load, which you know all about if you just tightened the belt properly.
The biggest problem is usually with the alternator not charging correctly. It is usually where the slippage occurs. It doesn't really cause damage per se, but rather the voltage irregularities might cause problems. IOW, as long as it stays on there and isn't squealing all the time, it should be good until you get it replaced ... replace it soon, though.
Something to think about in your situation, though, is instead of the belt being bad, you might be having an issue with your tensioner. Some tensioners are spring loaded (not sure about yours). If the spring wears out, it can make it so the belt will slip. Make sure you check this out when changing your belt just to make sure it isn't the root of your issue.
