I noticed that my idler pulley was bad, so I replaced it. Before replacing it, the pulley only made a worn-out bearing sound, not a squealing noise. However, after replacing the idler pulley and reinstalling everything, I noticed that my car now squeals loudly, especially at a cold start. I’ve already replaced the belt and the belt tensioner, but the issue persists. I can't recall if the idler pulley had a washer on the back of it, which might be causing the problem.

 

That's too hard for any of us to say, we can't know what happened.. You will just have to recheck your work. Remove the pulleys you replaced and look for evidence of rubbing. Make sure the tensioner has hard resistance and isn't defective. Warranty out the belt to rule that out. Look for a wobbling crank damper or misalignment from any of the pulleys.

 

The issue ended up being that they gave me a belt that was just a little too long so they replaced it with the gator belt that was possibly a half an inch smaller so when I replaced it nice tight fitting no more squeaking.