To the folks that have a 3.5 liter engine that has/had a noise that seems to come from the belt and belt tensioner please be advised if the belt and belt tensioner was replaced because of the noise (squeek-squeek sound) they should not replace those two parts but should replace the idler pulley instead. MB is instructing the service centers to just replace the belt and belt tensioner because of the noise. I have my belt and belt tensioner replaced already 2 times and the noise goes away then comes back after a few months. The shop head foreman said that MB is instructing them to unintentionally replace those 2 wrong parts and I think a lot of those fix have been coming back and they figured out it is the idler pulley which is the culprit.

So if you have your GLK fixed because of this issue keep an eye on it and document it when it squeaks (video with audio) and show it to the service center and tell them to look at the idler pulley instead. I was told that the head office needs a audio record of the noise to have the proper repair done or they will just have the belt and belt tensioner replaced.

Hmm.. there are 2 GUIDE pulleys... Is that what you're suggesting to replace? One is directly above the crank pulley, other is upper passenger side. Which one or both?

On mine the lower pulley was replaced, which I am guessing that would be the crank pulley. Part number 272-202-14-19. I was told sometimes both should be replaced depending on the premature wear on it.

Here's an interesting link to a DIY repair for the exact same problem.
http://www.slkworld.com/250749-post36.html

'shotgun' your thread brought great interest to me, as mine (2011) is doing the same 'squeak', but never replicated at the dealer!!! Changed belts, etc.
thanks...

I'm currently in the process of replacing the idler pulley on a customer's GLK350. We had the belt off for replacement and noticed some play in the grooved idler, not so much in the smooth idler or tensioner.

Ordered a gates replacement from amazon for $21, its marked Lichens (or something like that) and says Germany on it. You can definitely feel the tightness difference between the old bearing and the new one.

Car has 85k on it for reference and makes a very slight noise (I picked up on it as a possibility during an oil change and suggested we check during belt replacement).

I will post some pictures when I do the job tonight but its really very easy - remove belt (you dont even have to actually remove it, just drop the tension and keep it mostly in place). Unbolt idler, install new one to correct torque. Belt back on. Done - 20 mins.