Hey guys,


I've noticed a noise coming from my engine on my 1984 300D. The noise is only apparent when the car is cold, and even then it only seems to occur when the temp is kind of cold outside.

The noise sounds like a rattling. Almost like something loose smacking. Like if a bolt was backed out and a washer was smacking around on it. Haven't found any evidence of that, just a description. The noise does not go with the idle of the engine. It has no discernible pattern, but seems to be happening faster than the idle. When revved up the noise seems to subside, and when the engine warms up, the noise seems to subside as well.

It really scared me when I heard it the other day as my mind immediately jumped to rod bearings and things of the sort.

None the less I pulled it in while it was still cold to check it out. My girlfriend is a nurse and gave me an actual stethoscope to use, so I was able to listen to specific components. The first spot I put it on was the front of the block. I could hear the engine turning over, but the rattling was not coming through.

I got under the card and put it on a few different spots on the oil pan and still did not hear it. I also placed it on a couple of spots on the block, but there was no rattling/knocking in my ears. With how much this noise resonates throughout the engine bay, I think it would've definitely made it into the stethoscope had it been bottom end component issues.

So I started listening around various front engine components. This was prior to me researching the issue. I listed to the vacuum pump. It was noisy, but it was a constant, steady noise. I did not hear the rattle coming through the vacuum pump. However, I've read that this component can fail and cause this noise, so I don't know if maybe it is failing and the noise didn't make it into the Stethoscope?

I listened into the various other belt components and didn't hear much. But every time I just listened with my ears, it sounded like it was coming from the center of the front engine bay.

I put the stethoscope up to the water pump and swear I could hear a trace of the noise. The most I had heard from all the components.

EDIT: I did also listen to various points on the valve cover, but did not hear anything major from there.

So after researching, I've read this kind of noise is commonly found to be the fan clutch, vacuum pump or a timing chain guide.

Based on what I said about the noise I heard in the vac pump. does that likely rule it out, or could the noise just not be resonating through the case and thus not into the stethoscope?

And how could I test the fan clutch? Now that I read about the tensioner, I'm going to do some more in depth listening in that area tonight, but I'd love to have a couple other test I can try to hopefully narrow this down.


Thanks guys!

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1984 Mercedes-Benz 300D