Dear Rattled...
 

My '78 300D (The Gelb Flame) has developed an exhaust system rattle. I've replaced the front and rear rubber donuts, and the tranny bracket seems tight, and everything is clear of the driveline. I can change the resonance of the rattle with my hand idle. 'Seems like it is coming from forward of the tranny. Anyone among the great link of diesel wise have a suggestion? Thanks.

Have you pulled the air cleaner and checked the metal mount for it?

I replaced the mounts a couple of years ago with the new heat shield style, but I'll take a look. Thanks.

Look on top of your engine mounts...there may be a thin metal piece loose from the rubber and making the noise... my car has this accessory... LOL
A dab of silicone under it should fix it if that is the problem...

Had the same problem on my 80 300D. Started as a small rattle then got louder. Could make it go away with increased rpms. Unfortunately mine broke before I could figure it out. The flexplate that the torque converter is mounted on cracked and eventually broke taking the tranny with it. Remove the dust cover on the bottom of tranny by the flywheel to check. Good luck

I found it. :) The manifold to exhaust connection is quite loose, and the coupling is rattling. I'll pull the air cleaner tomorrow, rain permitting, and correct it. It looks like the wrong hardware too. As I remember they are supposed to be spring-loaded bolts to allow for expansion. I just see a couple of loose, and stipped bolts down there by flashlight.