Changed motor mounts and discovered this shimmer vibrational sound at certain RPMS. You'll hear it kick in around 40mph. I've also conjured this sound sitting still in neutral, revving the engine.

https://youtu.be/XzMQNq-w-XE

I've shaken everything in the engine bay and found nothing loose.
Disassembled and reinstalled the trans mount (new, recently replaced), made little difference.

My hunch, based on the quality of the shimmer, is something exhaust related?... any ideas?

__________________
1981 240D 4sp manual. Ivory White.

Sounds awful ''metalically'', almost like something bouncing around.

__________________
'80 300SD - '83 240D - '00 E55 AMG - '02 G500

Yes I think you’re right. Question is what…

__________________
1981 240D 4sp manual. Ivory White.

My SDL made an irritating metallic "rattle" and occasionally a very concerning "squeaking" noise at very specific RPM's. It turned out to be a piece of metal heat shielding was bent in the transmission tunnel and was *JUST* touching the exhaust pipe. Drove me nuts trying to find it, no amount of engine shaking or revving in neutral would show it up, it had to be in gear and moving for it to happen. Found it totally by accident.

__________________
Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

That sounds about right. The engine is riding a good centimeter or two higher, plausible something else is as well.

__________________
1981 240D 4sp manual. Ivory White.

Just had another pore over the exhaust, nothing loose there... possible it could be the drip/heatshield resonating? Anyone experienced that before?

__________________
1981 240D 4sp manual. Ivory White.

I can't wait to hear the solution, my '82 does this too since the engine and tranny overhauls ~ I thought it was the two year only center mount.....

__________________
-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

Same sound?

I did a semi-driveline refurbishment about 10 months ago -new center bearing, flex discs etc. This is just weird.

__________________
1981 240D 4sp manual. Ivory White.

Okay solved -was the driver’s side motor mount drip shield. Guess at certain resonances it started vibrating along with the arm mounted to it. I bent a corner down so it made more contact with the rubber mount and QED.

__________________
1981 240D 4sp manual. Ivory White.

Thanx for the follow up ! .

My hearing is shot to even with the sound turned all the way up I'm not sure if it's the same sound or not.....

My 240D vibrates at the same frequency as the sound I hear .

__________________
-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

Wow! Cool that it was something easy. It sounded like something more serious.

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

That was certainly part of it. Drove for 9 hours yesterday and heard it return around 70mph.

What's your theory?

__________________
1981 240D 4sp manual. Ivory White.

My 1982 240D's vibration is almost gone ~ all I did was a valve adjust, the valves on one cylinder were too loose, the valves on another were too tight .

Go figure .

I still feel too much vibration at idle but I'm not done fiddling with it yet .

I can't wait to find what you discover to be the true culprit .

__________________
-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

There’s a flap of metal bolted to the firewall that sits above the exhaust pipe. If you reach your left arm down from around the battery tray you can just hit it. Guess it’s a heat shield?
It definitely rings when you slap it. That’s my current suspect, though not terribly sure what to do with it.

__________________
1981 240D 4sp manual. Ivory White.

believe it or not my 240D had a rattling noise at idle in D that went away when I replaced a worn tie rod! I think it was so loose it was rattling at idle. Don't drive yourself chasing vibrations on a 240D. They a 4cyl diesels that make 65 hp.. they are gonna vibrate. Both of mine do and have had everything done to them.