My 1975 300D started making this noise about 2 weeks ago.
You'll notice it goes away any time the engine revs up. I only hear it when the car is standing and idling or when driving at low RPMs.

At first we thought it might be a loose exhaust pipe/leak, but once we got under the car the noise seems to come from the transmission. Someone suggested it might be the torque converter.

Anyone have any other suggestions based on the sound clips below?

https://soundcloud.com/fstop760/car-sound-1
https://soundcloud.com/fstop760/car-sound-2

Does the W115 have air cleaner mounts like the W123? They are prone to fail and make a racket when they do. That sure sounds awful but I don't think it's internal since it's not changing as you rev the engine.

Thanks!

The sound stops when I rev the engine and then comes back once I lay off the gas pedal.

I would have guessed exhaust leak from the noise, but you say no. I've never heard a sound like this from the torque converter. Maybe a stuck exhaust lifter or burned valve seat.

If it was something internal wouldn't it make the noise each revolution of the engine?

So once in a while, its frequency does increase as you start to rev the engine, and then as you go to higher rpm it stops. I would bet it is some resonance or rattle occurring because something is broken or loose attached to the engine. Air cleaner? Alternator?

My alternator mount on my 68 is different than yours, but my is cracked and makes a sound vaguely similar to that at idle. So maybe (hopefully) it is something external to the engine which just needs to be tightened or swapped out.

As an aside, will you promise to report back when you get this figured out. I am very interested in hearing the conclusion (no pun intended).

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1968 220D, w115, /8, OM615, Automatic transmission.
My 1987 300TD wagon was sold and my 2003 W210 E320 wagon was totaled (sheds tear).

Honestly, the first clip sounds like an exhaust leak/exhaust manifold leak to me.

However, the second one... hmmm... I have heard a bad water pump make that noise.

I would either buy a mechanic's stethascope or at minimum take long-shaft flat screwdriver and use it to listen at various spots under the hood.

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Mac
2002 e320 4matic estate│1985 300d│1980 300td
Previous: 1979 & 1982 & 1983 300sd │ 1982 240d

“Let's take a drive into the middle of nowhere with a packet of Marlboro lights and talk about our lives.” ― Joseph Heller, Catch-22

Sounds like a loose heat shield but I cannot think of any on that car. Also might be the vac pump.

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

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strange sound....I would check flex plate ,...get under and put a hand on TQ
body/place so you can feel is it a source... you will need another guy on pedal
to rev a bit engine up and down to idle....


Poll: Have you ever heard/experienced a broken flexplate '80 300D

cheers
ChO

.

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w126 500SEC gen II euro, powered by OM617 turbo stolen from 84 300SD 2.88 diff,EGR blinded
next wish/project: w114 coupe OM603 powered