A weird murmur at exactly 10 on the tach 84 300SD

Hi,
This has got to be the runner up in the "weird things that happen to my old Benz" contest.
I have recently noticed this weird sound coming from the rear of my '84
300SD. At first I thought it was an empty gas can shuffling around in the trunk, but then I looked at the tach and realized that the quiet, yet deep and bassy sound coming from the back happened only when the tachometer showed 10. Not 9.5, and not 11, but precisely 10.
I could make the sound appear and disappear by gently playing with the gas pedal. It's unsettling.
Thanks!
Mia
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Mia -
Your tachometer has found the resonant frequency of something loose on your car. Try tightening anything you find loose in the general vicinity of the apparent noise source. If you're a true scientist you'll perform this one at a time, then go test drive. When your tach suddenly shows *other* than 10 when the noise appears, then the last thing you tightened was the noise source (or something close to it). Good luck in your quest . . .
Jerry Wolfram '78 450 SL - 214K miles
Mia wrote:

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Jerry,
I briefly looked around for things to tighten, nothing yet. I did find out that when idling, the air filter casing shakes like mad, even though the engine is barely vibrating. You know by any chance how to tighten the air filter casing?
Thanks!
Mia
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Harmonic vibration from the rear - check the exhaust pipe hangers - rubber donuts that support the pipe and muffler. Also check that the spare is tightened down in its well and that the pressboard cover over the well isn't rattling.
Air cleaner. That's a sore subject on this motor. In 109K miles my car has broken three support brackets - always in the same place - the rear leg. It just can't take the sideways oscillation of the big air cleaner. I had a reinforcement piece welded to the last new one. Then I added a piece of heater hose as a bumper between the air cleaner and the valve cover to snub the oscillations. If the reinforced bracket breaks I'm going to mount the air cleaner on the body rather than the engine.
The air cleaner can is mounted on three rubber mounts. These break, usually the rear one sometimes the outer (right side) one. They aren't cheap, for what they are, but there's not much choice but to buy some and replace them as needed, not sooner.
Ready for the cold weather? Got those valves adjusted yet?
Tom
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I don't think you would want to do that, as the air cleaner needs to be able to move with the engine, or something even more expensive/necessary will break....

significant number of these are pre-broken at the bone yard?
Marty
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you can find some good ones at the unk yard, but eventually they go bad, if you want new ones, the cheapest i found are http://parts.mbz.org/ they have very good prices, and i have talked with the guys that work there on the phone (i live in Florida) and also emailed them, the guy who runs it, Rusty, has been a MB diesel mechanic for something like 35 years, extremely helpful, even if you dont buy anything. I spent like 2 hours on the phone with him troubleshooting an issue i was having with my Dad's '82 300SD with the Air cleaner.
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