jdsarahj

I have a 2011 C300 4matic with about $65k miles on it. Earlier this week I started noticing a ticking/light tapping kind of noise when I take my foot off the gas and just let the car coast. If I throw it into neutral, it stops. If I barely even touch the gas, the noise stops. It only happens when I take my foot off the gas and let the car decelerate. The tapping sound also seems to match the speed of the car - so the slower the car goes, the slower the tapping seems to get. It doesn't make the noise while in park while pressing the gas. I took it to a shop down the street yesterday and the guy looked at it for 15 minutes and told me it's the idler pulleys that need to be replaced. The sound that people have described when those are bad (whistling/whining noise) doesn't match at all what I'm hearing. Any thoughts before I take it back to this guy to get these replaced?

I took a video earlier so people could attempt to hear the noise. I'm sorry it's so quiet. The sound starts right around the 16 second mark -

rogerl19

Thanks for posting the Youtube video. That's the same noise that I'm getting on a 2011 E350. It's been happening for years but never did anything about it. Did you ever figure out what was causing it? Thinking of taking it to the dealer and having them listen to the Youtube video. It's really annoying and I've checked every loose pieces under the hood but can't seem to find it.

Funkwagen

I had a noise almost exactly like this, except it was more quiet and could only really be heard at a stop. It was the idler pulley.

You can take off the belt and try to spin the pulley by hand. If it wobbles or doesn't move freely, you'll know you need to replace it

Also, check your belt tensioner

jdsarahj

I took it into the dealership shortly after the noise started - they told me it was the U-joint. Luckily the car is still covered under the certified warranty. Otherwise it would have been $1570. But they replaced it and no more noise. The car was actually making a pretty good *clunk* when shifting from park to reverse/drive which was also fixed by replacing the U-joint.

rogerl19

Thanks for the insight. Brought it into the dealership today and took a test drive with the shop foreman. We both heard the sound and he also noted that it was coming from the drivetrain. I thought it was coming from the front of the car, but he had me switch places with him. When we drove, I could now hear that it was coming from the rear underbody. I told him that your dealer's service dept found and fixed the U-joint, so maybe that'll save him time if it's the same thing. Not under warranty anymore Will let you know what they come back with.

rogerl19

Believe it or not, got my car back about two days ago. I told them to take their time and man did they take me up on that offer.


They found the problem to be in the planetary gear box in the differential. Seems like my dealership's service center spoke to another expert in MB and pointed them to this part. According to the service invoice, they replaced the "Transfer clutch pack".


I've been driving it around as much as I can and so far the noise is gone.


The total came to around $650 which included some things they recommended which didn't fix the problem, but had to be done anyway at some point (like trans mount replacement and tire balancing).


Thanks for the post and replies. The clip really helped me convince them of the problem!