is this an alternator noise or something else? (vid)

[rh71]

Just brought the car in for annual oil change/filter & inspection with no issues other than a small transfer case leak that they say doesn't need to be addressed yet.

And now on random cold starts it produces this noise for 35 seconds then goes away - then the belts normalize in speed. '10 C300 with 15k mi.

http://youtu.be/m57seZoYyII

 

[Wingznut]

Pretty hard to tell from an internet video... But I'd say it's much more likely to be an idler or tensioner pulley than an alternator. Also very possible that it's the belt itself. My money is on the belt, since a bad bearing in one of the pulleys doesn't usually go away, especially that quickly.

A simple way to check the belt, is to give a quick (very small) spray of WD-40 on the belt itself while it is making the noise. If the noise changes, or goes away, it's the belt. Be careful to not spray too much lube on the belt.

If you want to diagnose which pulley, you can buy a mechanic's stethoscope and listen closely to the components. You have to be so incredibly careful though, sticking that thing around a moving belt and pulleys.

 

[rh71]

I think I'm going to use water and see what happens. I don't think it's the belt because of how mechanical the noise is - almost like it's inside a component. But I'm no mechanic.

And just curious, what is with the vacuum-whine at around 0:36? Was it struggling to get to speed initially then finally is spinning freely? Spun up and then back down over a span of 6 seconds.

Any way it's related to an oil change being done because it's literally the day after, this started happening.

 

[Raizinman]

Many Mercedes have an oil filter that is mounted high and in the front of the engine. I've heard of some cheap oil filters that drain back and for the first 10 or 15 seconds, the oil pressure must build up pressure to the filter and that is that gurgling sound you are hearing and then goes away. I would think that when you leave the car for a few hours, the noise might return. to fix is to just change out the oil filter. There are numerous o'rings that must be changed with each oil change. Often non-Mercedes shops rarely change the o'rings. Just a thought.

 

[rh71]

^ newb question - can I remove the oil filter cap in the engine bay even if the oil isn't drained from the bottom first? I just want to inspect the filter to see if there's anything suspect with it. No issues like tank pressure or introducing air that I have to worry about? I was watching this: https://www.youtube.com/watch?v=aNvfRHuZTFY#t=147

Curious - if it is the filter, is it damaging to the car or can we live with it till next filter replacement?

 

[Wingznut]

I think I'm going to use water and see what happens. I don't think it's the belt because of how mechanical the noise is - almost like it's inside a component. But I'm no mechanic.

I'm no longer a professional mechanic, but I was for a dozen years. And yeah, I've heard belts make noise like that.

Can't say that I've ever tried putting water on a belt, not sure why you would do that instead of a lubricant. It's possible that water will just cause a different noise, and not solve anything.

 

[Raizinman]

^ newb question - can I remove the oil filter cap in the engine bay even if the oil isn't drained from the bottom first? I just want to inspect the filter to see if there's anything suspect with it. No issues like tank pressure or introducing air that I have to worry about? I was watching this: https://www.youtube.com/watch?v=aNvfRHuZTFY#t=147

Curious - if it is the filter, is it damaging to the car or can we live with it till next filter replacement?

Yes, you can remove the oil filter cap and pull out the filter. I would suggest that you stop by the auto parts store and purchase a new oil filter. It will come with all sorts of o'rings in the oil filter box. Often there will be instructions in the oil filter box. Just go ahead and replace the filter to make sure.

One other note: Because your oil filter is high up on the engine, lazy mechanics will pull out the filter and let it drip all over, including on the belts, but I didn't really hear a belt type noise.

 

[rh71]

I just took off the cover and the noise is coming from this thing right in the middle of the pic... identified as the air pump... OMG why is the tube disconnected?

 

[LTC8K6]

Connect the tube.

 

[Wingznut]

I just took off the cover and the noise is coming from this thing right in the middle of the pic... identified as the air pump... OMG why is the tube disconnected?

Yep, that'd do it.

 

[rh71]

Car is fine now. Would you guys put it past the dealer that they did this on purpose? Bringing a car in just for diagnostics is no less than $75.

We're out of warranty and I only used them because of a coupon for the oil change that saves $40, and a taillamp recall.

 

[LTC8K6]

Too obvious.

 

[sontakke]

You need to make huge stink; was there even a reason for the technician to go in that area? Somebody should lose his job over this. This is not an honest mistake but rather deliberate sabotage and done by somebody who knows the system well. I suspect the airpump only runs when the engine is cold on the first start. And thus the problem would NOT show up when you picked up the vehicle from the dealer.