We just purchased a 1979 300SD with 224,000 miles. The rear end had a loud clunk noise. We had a Mercedes mechanic check the rear end and he said that was fine for the amount of miles. When you accelerate between 50 & 60 mph you get a loud humming noise from the rear end. This only happens on ocassion.

Thanks for your help.
Colleen

You are generally describing what sounds like excessive clearance between the ring and pinion gear. The clunk is usually a dead giveaway. Humming could be a bearing, but most likely it's the ring and pinion clearance. This will need some attention soon.