I've had an issue recently surface with my 1978 300D. Occasionally I get a pretty intense WUMP WUMP WUMP that shakes the car a bit, but if and only if the engine is fully warmed up (temp just above 80 and oil pressure at 2 bar), it's in Drive, and it's not moving (i.e. stopped at a red light).

It only happens every so often, and if it's happening and I put the car in neutral or reverse or park it stops. If I take my foot off the brake and start moving it stops. It sounds and feels like a bowling ball in a cement mixer under the hood. Otherwise the idle is pretty smooth.

The car drives and shifts normally. Oil level and trans fluid is normal.

My brother in law is a diesel mechanic. We drove around all day Saturday in the car and of course the car behaved itself the bugger. My brother in law hazarded a guess that maybe air is getting in the fuel feed somehow but without seeing the problem first hand couldn't really offer an diagnosis. I have one slightly loose injector return line (the braided hose) that seems a little damp, other than that I see no fuel leaks.

It's a non turbo 617 engine. I'm not sure what the deal here is or how to solve it, any advice would be greatly appreciated.