I have a mysterious buzz-rattle
 

I recently inherited my Grandfather's 1976 Mercedes diesel 240. I love this car except for one thing: it rattles/buzzes very badly if you drive it at low speeds in high gear. Could this be a problem with the transmission? How much does that cost to fix? I guess I could drive in third gear in town but, it seems like at 40 mph the engine is racing and wants to be shifted to fourth.

Thank everyone in advance.

John