I have about 89k miles on the car and was experiencing really loud squealing noises at idle, going reverse, and when the A/c is on. So i took it to the shop and they ended up replacing the belt tensioner assembly, as well as the drive belt while they were at it.

That solved the problem completely for about 200 miles but now the noise has returned especially when the A/C is on. Has anyone else experienced this, and what was the underlying issue? I am curious what to expect when I take it back to the shop this week.

The high pitched squeal is coming from the drivers side area and is pretty embarrassing especially when I'm stopped at a red light.

 

They may have used a cheap belt. If a quality belt isn't used, they have a tendency to stretch and slip, causing squealing. Another thing check is the idler pulley. and the AC pulley, it could be that the bearings in them are going bad.

 

Check your main pulley and see if it's separating from the center within itself. Happens on older toyotas a lot...causing this squealing noise rendering people to question the belts mad tensioners. To inspect it, you'll have to go from the side of the pass tire, can't inspect it from the top.

Kruso

 

Squealing with the A/C on is its compressor pulley going out. Throw some WD-40 on it and you'll see that it will stop momentarily if you can get it all the way in there. It's like two bolts to replace it.