I have an 84 300d. Over the past year a "grumbling" has begun in the rear of the car. It sounds like a large spring rattling around esepcially when I go over rough pavement or gravel. It does not happen when I go ever a speed bump and seems worse the faster I go.
I thought it might be the exhaust, but it has all of its dampers and does not seem to be rubbing anything.
What can I look for to find these gremlins?
I am not sure if it is under the car or in the trunck, but I am nearly sure that it is under the car.

Check rear suspension bushings for slop and rear sway bar links. Could also be worn shocks- Mine made alot of weird noises until I replaced them.

__________________
'83 300D Turrrbo 295K miles

check to see the top of the rear muffler isn't touching the body near the rear bumper, there's a rubber cushion that is there to prevent that. I've seen a exhaust line that was bent up higher than it should and made a grumbling noise where it rubbed against the chassis.
Also an odd noise can come from a fuel sensor inside the fuel tank that lost its cover. That is usually a noise that happens when cornering. don't ask me how I know what this sounds like ! (and I hope that didn't happen to you!)

__________________
'95 E320 Wagon my favorite road car. '99 E300D wolf in sheeps body, '87 300D Sportline suspension, '79 300TD w/ 617.952 engine at 367,750 and counting!

I second checking the sway bar links. My 240 is making that same noise now and I have 2 new links to put in.

__________________
My Truck.. 2007 DODGE, 5.9 Cummins, 6spd stick, 4X4. My car..1977 240D, OTHER WHEELS...1955 VW Oval window bug, European Delivery (Holland) with a 1700cc, 2 barrel, Porsche drum brakes. 1939 WILLYS Pick-up. 1967 Triumph 200cc Tiger Cub. 1976 Honda 550F 4cyl Motor Cycle.