Squeaking noise at idle, 300SDL
 

Ever since I replaced the serpentine belt shocks I've had this squeak. At first I was afraid that I had abused the water pump shaft and it would soon leak. It hasn't.

https://youtu.be/4s-sEa4Xnq4

Some of you guys recognize every other sound so who knows.

Yay I know! Well because I've had this issue multiple times.

Your belt is not centered on the idler pulley, it's either too far forward or too far behind.

The tensioner is funny, you'll have to push the belt BACK a little further than you think before re-appling tension because it will ride forward when tightened.

Ok so here's how you fix it, loosen pulley with lug wrench, slowly and gradually lower the lug wrench with one hand and adjust the belt with the other. Once in the desired spot let it go.

If you let go to early it will ride forward because of because of slack, if you let go too late in the wrong spot you have to start over.

It's a balance of adjusting the belt with pressure of the tensioner

Not sure what the previous poster is carrying on about, if the belt were off-center when you levered up the tensioner, it would self-center when you cranked the engine.

More than likely you have a worn tensioner pivot or tensioner pulley which is allowing the belt to scuff as it rolls across the pulley since it's out of alignment.

Remove tension and check for any play in both the tensioner pivot and the pulley. It wouldn't be unheard of for the tensioner pivot to have had it after chattering like it was with the bad shock.