Sigh...another problem for the C280...

Last year, I noticed the Poly-V belt was getting a little worn out, so I had it replaced by a garage (bunch of other things had to get fixed). However, all winter long it would squeel when I started the car. I hoped it would go away when the weather got warmer...but it hasn't. Could it be that the tensioner pulley wasn't tightened to spec? I tried spraying the belt dressing to try and quiet it, but it didn't work either. Any ideas?