For the last few months I have been trying to find the source of a knocking noise in my wife's 1981 300SD. It was temperature and RPM dependent. It occurred most noticeably when the engine was less that normal operating temp and the RPMs were around 1,500 and 1,800.

It sounded just like it was emanating from the area of the left resonator. I even took the rear seat bottom out and rode a friend around who agreed with me that it sounded like the left resonator. You could all but feel the noise under the rear seat. Since I also have a 1982 300SD, I simply swapped the whole exhaust systems. The noise was still there. This troubleshooting process went on for months, trying other things which might cause a noise like this.

I have also owned a pair of 1978 300Ds and had to clean their fuel injectors about once a year. The longitudinal and transverse holes in the pintle would get clogged and cause a remotely similar, but not exactly the same noise.

I cleaned the injector nozzles in an ultrasonic cleaner and lapped all surfaces. I was rewarded with quiet engine operation again.

I had heard about oil drips dropping far from their source, but never a noise emanating far from the source.

I should not have been surprised. I ran across a passage in the Bosch Automotive Manual (3rd edition) which states this about diesel injection:

"Thus the processes attendant upon injection are not static, ... but are instead dynamic (essentially according to acoustic principles)."

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Tom Evans
69 280SE 91K (wish I still had it)
73 220D 98K
78 300D 134K
78 300D 185K
81 300SD 301K (still have)
82 300SD 321K (still have)
84 300D 268K
84 6.2 Diesel GMC Suburban 225K (a whole different story!)

Wow... Perhaps the noise was traveling out the exhaust, and reverberated in the resonator ?

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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 190D 2.5Turbo
1987 190D 2.5-5SPEED!!!

1987 300TD
1987 300TD
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!

That what I thought. And it was the left resonator with both exhaust systems.

__________________
Tom Evans
69 280SE 91K (wish I still had it)
73 220D 98K
78 300D 134K
78 300D 185K
81 300SD 301K (still have)
82 300SD 321K (still have)
84 300D 268K
84 6.2 Diesel GMC Suburban 225K (a whole different story!)

This doesn't sound too far-fetched to me. I've currently got my entire exhaust system dropped, and have at least 2 glow plugs which happen to need replacing... (don't ask)

So, while the engine is still running cold, the misfires coming from a couple of cylinders are pretty easy to hear. It's very easy to imagine those pressure waves terminating in one of the resonators - banging in it, if the system were installed.

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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

Two ways to scope out the RPM related noise source:

1st, put the car on a rack and tach up to the RPM area in question. If the noise is related to strictly engine RPMs, it will reveal itself while on the lift.

If that doesn't reveal the noise, then it may be drivetrain-related. For this, you will have to employ the use of a dynamometer to locate the source of the noise. With this method, the entire drivetrain is 'in-motion,' so, it should make short work of determining and pinpointing the noise source(s).