Serpentine belt noise at cold startup?

Tiny
JITHIN MATHEW
  • MEMBER
  • 2011 MERCEDES BENZ C300
  • 3.0L
  • V6
  • 4WD
  • AUTOMATIC
  • 109,600 MILES
I’ve been dealing with my belt making noise on cold startup until the engine warms up a little for couple of years now. I’ve changed multiple parts on it and it’s always a temporary fix. I changed the belt tensioner twice, upper and lower idler pulleys 3 times, and the drive belt multiple times. I have used both OEM and aftermarket times and it still hasn’t fixed the issue. I checked all the pulleys for play or any noise and they are quiet. I had a Mercedes specialist also look at it and they could find anything wrong and therefore recommended a new belt. I bought the Mercedes belt and installed it and there was no noise for about 1,000 miles but then after that it started again. It also does it when I do a cold startup and until the engine warms up a little bit because after that then it’s completely quiet. There was 2 TSB’s issued for this issue in the M272 engine and I followed both of them. One of them was to apply a krytox lubricant on the edge of the belt and run the engine. That quiets the belt down but after the lubricant wears off it starts making noise again. 2 weeks ago I had to change the thermostat, so I changed the oil filter housing gaskets and the upper coolant hose o ring because there was a seepage. After cleaning all the pulleys it got a little better but it’s still making quite a bit of noise. I don’t think there is pulley misalignments either because the noise stops after about 5 minutes of driving. Any suggestions?
Thursday, March 27th, 2025 AT 11:26 PM

13 Replies

Tiny
KEN L
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Yep, I had this problem with my wife's c230. So, Mercedes makes an alternator that has a one-way clutch on the front of the alternator which stops this noise, by chance has the alternator been replaced recently? This clutch stops the alternator from being subject to engine vibrations which makes the belt produce the noise described. Please let us know what happens.
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Friday, March 28th, 2025 AT 10:46 AM
Tiny
JITHIN MATHEW
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I’ve had this car since it had 63,000 miles. I believe the alternator is still original to the car and has not been replaced, at least to my knowledge and according to the service records. My independent Mercedes specialist said there was a slight play in the alternator pulley. I just don’t understand how the belt noise completely goes away after the engine warms up a little bit. I know the alternator is at higher demand on cold startup. The belt is for sure slipping because every time I clean the pulleys, my rags come out black as well as the belt.
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Friday, March 28th, 2025 AT 3:22 PM
Tiny
KEN L
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Can you check the tension on the belt? You could have a weak belt tensioner. Can you upload a short video of you checking the tension so I can see?
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Saturday, March 29th, 2025 AT 10:29 AM
Tiny
JITHIN MATHEW
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  • 71 POSTS
Attached is the video is me checking belt tension. The tensioner is only a couple months old and it’s a gates tensioner. I thought the tensioner was bad which is why I changed it but that’s not culprit. The belt noise does depend on the temperature too. When the car is parked inside the garage with the ambient temperature around 60-70 degrees Fahrenheit, the belt makes noise on cold startup but goes away after like a minute of the engine running. When it’s parked outside for 10 hours and it around 32 degrees then the belt makes noise until it goes away completely about 5 minutes into actually driving time of the car.
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Sunday, March 30th, 2025 AT 12:17 PM
Tiny
KEN L
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It looks like you have good tension on the belt, I would remove the belt and check for a lose water pump pulley etc. Something must be on the pulleys, or there is a crack that is causing this. Thanks for the video, can you upload one with the engine making the noise?
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Monday, March 31st, 2025 AT 11:30 AM
Tiny
JITHIN MATHEW
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I have checked for loose pulleys so many times from the very beginning when it started making noise and I checked it last week as well. They are all tight and have no play in them. My Mercedes specialist did tell me that there was a slight play in the alternator pulley but it’s nothing to be concerned of at this moment. I just don’t get this car. It all started when I was trying to do some preventative maintenance. I guess if it isn't broken then don’t fix it. I will try to get a video for you when I can.
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Monday, March 31st, 2025 AT 1:53 PM
Tiny
KEN L
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Like I said, I had this problem as well, it was the alternator. I have to update it to the one-way clutch model. It must have something to do with the engine wear.
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Tuesday, April 1st, 2025 AT 8:58 AM
Tiny
JITHIN MATHEW
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Good to know. I attached the video of the belt making a lot of noise on cold start up. The car was parked inside the garage. So does these cars come with alternators that’s doesn’t have a clutch and we have to upgrade it? I also have the 2390 mm installed on there currently and that is what you see in the video.
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Tuesday, April 1st, 2025 AT 2:35 PM
Tiny
KEN L
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Thanks for the video. This noise is not the alternator or the belt, it sounds like the belt idler pulley bearing is out, I would remove the belt and spin the pullies and check for play to see if you can find it, you may be able too without tension on it. Did you replace the most upper pulley?
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Wednesday, April 2nd, 2025 AT 10:47 AM
Tiny
JITHIN MATHEW
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Yes, the belt, upper and lower idler pulleys were replaced multiple times. I checked the pulleys, and they are smooth and quiet with no play.
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Wednesday, April 2nd, 2025 AT 12:48 PM
Tiny
KEN L
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It can be the water pump or the AC compressor pulley bearing. It does not seem like a belt noise to me. If you spray WD40 on the belt does the noise go away?
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Thursday, April 3rd, 2025 AT 9:30 AM
Tiny
JITHIN MATHEW
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There is a Mercedes specified lubricant that was published on one of the TSB’s which is what I usually use. Every time I put that onto the belt, the noise goes away until it wears off again. But any liquid sprayed onto the belt instantly makes the noise go away which is why I don’t think it’s a pulley issue, but I don’t know. Also putting a brand-new Mercedes belt on will keep the noise away until the belt has 500-1000 miles approximately and then it starts again.
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Thursday, April 3rd, 2025 AT 3:28 PM
Tiny
KEN L
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I am not sure what to tell you on this one. There must be one pulley that is causing it, clean the grooves and looks for small rocks imbedded. Are any of the pulleys hard to spin? Does the power steering work okay? With the belt removed the noise goes away, correct?
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Friday, April 4th, 2025 AT 10:16 AM

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