Own a 1979 300sd w116 and for the longest time I have been hearing this clatter/rattle under the hood while the car idles. I seem to hear the rattle/clatter a little more lately.

When hitting the accelerator the noise goes away.

When I slow down to reach a STOP, the clatter is immediately heard and as soon as I tap on the accelerator the clatter/rattle noise goes away.

I know this is not normal.

I'm sure someone here knows where the noise is coming from

Please advise

Of course it may be because it may be seized up but the compressor clutch on my brother's 1980 300SD rattles like that.

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Current: 1975 450SEL, 83 300D, 88 Yugo GVX, 90 300D OM603 swap, 91 F150 4.6 4v swap, 93 190E Sportline LE 3.0L M104 swap, 93 190E Sportline LE Megasquirt, 03 Sprinter, 06 E500 4Matic wagon.

A video of the engine running with sound really helps in cases like this.

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2004 F150 4.6L -My Daily
2007 Volvo XC70 -Wife's Daily
1998 Ford F150 -Rear ended
1989 J-spec 420SEL -passed onto its new keeper
1982 BMW 733i -fixed and traded for the 420SEL
2003 Volvo V70 5 Speed -scrapped
1997 E290 Turbo Diesel Wagon -traded for above
1992 BMW 525i -traded in
1990 Silver 300TE -hated the M103
1985 Grey 380SE Diesel Conversion, 2.47 rear end, ABS -Sold, really should have kept this one
1979 Silver 300D "The Silver Slug" -Sold

I had a 1983 300SD with a similar rattle at idle speed. The balancer/pulley on the front end of the crankshaft was loose. We couldn't determine the cause of the noise until the balancer/pulley stopped turning while the driving the car at highway speed. A/C quit on a hot day, engine temperature rose, hard to steer----flatbed tow truck.

AC compressor is inoperable, no belt.

The t-shaped Air Filter Bracket often cracks on these cars. That air filter can rattling around made my otherwise pretty quiet SD kinda noisy! She announces her arrival! And, yes; this can be engine speed-sensitive - different frequencies, different vibrations.

Replacements available, or it's a simple weld/braze to fix.

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'83 300 SD
'68 Triumph TR 250 - The only car I ever loved more than the Mercedes; who needs electricity, anyway? - Damn, why did I sell it?!
'59 Jaguar 3.4 'Le Chat Noir' - Damn, why did I sell it?!

It's difficult to make predictions, especially about the future.
- Niels Bohr

I also have a 1979 300sd (w116) and it did the same thing. I traced it to the front exhaust mount to the transmission. It seems the parts for this are hard to find/expense so I have put it off. The rubber bushings in my mount have disintegrated causing the rattle at idle. One more thing to check.

I had the same issue. The problem was the air filter housing. I changed the broken rubber /screw elements 2x (within 1 year). Then a friend came by and said that the best idea is to change the RPM in idle. That's what we did and since then the air filter housing never broke the screws.

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'80 300 SD, 300k

Had the same problem....it was loose engine shock...tighten up the 10 mm nut.....now ok

I'm gonna vote again for the Air Filter - what was the final diagnosis?

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'83 300 SD
'68 Triumph TR 250 - The only car I ever loved more than the Mercedes; who needs electricity, anyway? - Damn, why did I sell it?!
'59 Jaguar 3.4 'Le Chat Noir' - Damn, why did I sell it?!

It's difficult to make predictions, especially about the future.
- Niels Bohr

Ha! I had the same problem too! I put it up on a lift about 2 years ago, had someone do a power brake (stomp on the brake and gas simultaneously) to isolate the noise, and it turned out to the the exhaust mount just like johnnyk6's problem. Tightened as much as humanely possible and it went away.

Fast forward 2 years, I left my baby sitting in the garage for a couple months while on a work trip, and start it up to find the noise is back, but worse.

My condolences; it's a rather annoying problem, I know. Hopefully it's the air filter housing, which would be a waaay easier fix.

If its not any of the above likely sources, possibly the fan clutch.

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1959 Gravely LI, 1963 Gravely L8, 1973 Gravely C12
1982 380SL
1978 450 SEL 6.9 euro restoration at 63% and climbing
1987 300 D
2005 CDI European Delivery
2006 CDI Handed down to daughter
2007 GL CDI. Wifes

i have this problem on my 300D, but it has the 722.120 trans which is usually found in your w116s, coming from the transmission exhaust mount. i've "fixed" it once but it returned. i wonder if it is an inherent flaw in the design.

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1981 NA 300D 310k miles

Here's a video explaining the rattle, might be your case:
https://www.youtube.com/watch?v=190vxFy-iY4

There's an after-market heavy-duty version; time to upgrade!

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[GONE] - 1995 Mercedes E300 Diesel - 130k miles - Smoke Silver (702) over Mushroom leather (265) - Bladder blasting, coast-to-coast work machine.