At idle in my OM603 powered 1987 300TD there is a loud interior rattle that sounds like it is coming from behind the driver side dash (It's definitely driver side and not the interior temp fan thing that is behind the passenger side dash, i checked).

When applying throttle in park or neutral the noise becomes less severe, and when driving it isn't noticeable. When I shut the engine down the noise continues very briefly past the point of the engine being off. Sometimes it almost sounds like a noisy fan spinning at low speed.

I've tried locating the sound from the engine bay side but haven't been able to find it. If i put my ear up close to the brake booster I can hear it faintly, but I don't think a failing brake booster makes a noisy rattle.

I also thought that maybe it was the speedo, but the noise is worse when not moving so I doubt that is the case.

I plan to get into it at some point soon by removing the gauge cluster and possibly the lower panel, but I was hoping someone might already know what this could be.

I'm keen to hear what you find ~ two of my W123's have similar rattles, no spinning sounds though .

The third one made nary a sound in spite of breaking lots of air cleaner 'T' brackets, I found loose 10MM ATF bolt way deep in there, it was hard to reach but I snugged it up bit by bit .

Let us know what you find .

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-Nate
1982 240D 408,XXX miles
Ignorance is the mother of suspicion and fear is the father

I did then what I knew how to do ~ now that I know better I do better

On American Cars I would say that sound is from the Speedometer (gears inside) and or Cable. If there is some area where you can get your hands on the Cable you may be able to feel the vibrations.

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84 300D, 82 Volvo 244Gl Diesel

Does it appear to be coming centered around behind (inside)the dash somewhere below the loudspeaker and behind the light switch?

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'83 SD, 2x '85 SD
You are entitled to your own opinions, you are not entitled to your own facts.

Yes, that sounds about right

Hard to imagine how the speedo cable can make noise when the car is stationary and the engine off.....

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-Nate
1982 240D 408,XXX miles
Ignorance is the mother of suspicion and fear is the father

I did then what I knew how to do ~ now that I know better I do better

Agreed. The previous poster described where the sound is coming from very well:

I'm hoping there will be a follow up to that question

Let's try recording the sound and post it here to see if it's the same.

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'83 SD, 2x '85 SD
You are entitled to your own opinions, you are not entitled to your own facts.

I had an annoying squeak / buzz / rattle that I too thought was behind the driver's dash but really wasn't. Took me about a year to find it.

It turned out to be the shifter bushing under the car at the transmission. The bushing was worn away. After replacement the car was quiet as a church.

Shift link bushing

Shift link bushing clip

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1999 Mercedes E300TD daily driver sold at 238K miles 106K miles were mine, rust worm got it :-(
2006 Mercedes CDI new daily driver! 56,000 miles May 2016 now 85,625 Apr 2018 and Apr 2019 101,000 miles Apr 2020 109,875. March 2024 135,250

Update - I pulled lower dash panel and now I don't think it is coming from in the dashboard (as I think several people already suspected).

Still haven't gotten a recording.

I did notice that I *think* the noise goes away when I go over a speed bump. I.e. if I'm at idle (which is when I hear the noise, it goes away when giving a little bit of throttle) but rolling over a speed bump it sounds like the noise stops for a second as the front wheels go over the bump. This could totally be my imagination, but I did replace the struts so I'm going to look if anything is loose or wrong on the drivers side strut.

Bad vac. ck.valve can make plenty of noise,some models have one behind the cluster.

I still haven't tracked this down. I'm planning to replace my motor mounts soon and will see if potentially what I'm hearing is related to just engine vibration

I replaced my motor mounts this last weekend and the noise is now gone (not to mention i can barely feel the engine at start-up and shut-down).

What I don't know for sure is whether the new mounts stopped something from shaking/rattling that was causing the noise, or if it was the mounts themselves. I suspect the latter because I can hear something rattling inside the old mounts when I shake them (I'm pretty sure they were the original mounts).

I was going to suggest might be fan clutch.

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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.

Ill toss in my diagnosis. I don’t think it’s a speedo cable...aren’t the 124s electronic? I guess they have a motor in there.

Anyhow I have had buzzing and whirring from my CCU in three of my mb diesels. It was caused by relays opening and closing at an audible frequency so it made kind of a tone so to speak like a fly flapping its wings. I just fixed this this month in my SD by pulling the CCU and resoldering it’s bad solder joints. On visual inspection there were at least five cracked solder joints (ring around the solder bump from 38 years of thermal cycling). Free fix. This has worked in my cars.

Somehow the shutting down and listening to it slowly go away blasts my theory.

Could it be a defective water temperature valve? Like a squeaky faucet? Doesn’t the 124 have a more sophisticated monovalve than the 123? That may explain the bleed down of the noise. It can also be something jammed in the blower?

Good luck chasing it down.

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79 300TD “Old Smokey” AKA “The Mistake” (SOLD)
82 240D stick shift 335k miles (SOLD)
82 300SD 300k miles
85 300D Turbodiesel 170k miles