U-joints. Get a hammer and a punch. Replace both ends of the drive shaft.
You're a clunk-head...
Sway bar bushings
Check for Rheumatoid arthritis. Cervical spine X- ray.
You have either run something over or your car is due for a service
One of the motor or transmission mounts? Does it clunk when putting in gear from park?
The Transmission needs SERVICED A.S.A.P Hope This Helps.
Worn CVC joint. Could be bad struts causing the spring to bind. Depends how severe the clunk is. If it is struts, it sounds like the floorboard is getting hit by a sledge hammer.
I had the same problem. The dealer replaced the engine mounts at $550.
It could be that the transmission fluid is low, or it could be that mechanical parts have worn down. If so, it may be time for a replacement.
The simplest thing to check would be the universal joints on your drive shaft. See if you have any play in any of those universals. The springs in the center of the clutch disk could also be out and cause a clunking. MGB's also have thrust washers in the differential that wear out and cause a clunk in the differential.
The first thing that comes to mind if the "clunk" is only on starting is worn or broken motor mounts. Something is moving and allowing things to hit each other. A clunking that is intermittent while running may be a ball or roller bearinng going bad.
Vrrrrrrroommm If the driver puts his foot down. Brrrr-clunk-rrrrr-clunk-rrrrrrrrr With added gear changes.