Possibly a worn out universal joint.
Could be a broken motor mount Check exhaust system for broken hanger
Year, make, model, engine size, etc. does help. Possible engine mount, u-joint differential,trany, etc...
Are you sure it is the transmission that is clunking? It could be the u-joints on the drive shaft that are making the noise. When the u-joints on the drive shaft are bad, there is an audble clunk when you accelerate and when slowing down (basically the same motions you go through when shifting gears).
You're a clunk-head...
Usually it sounds like a clunk. Put the car in drive, step on the brake pedal and hold it down while you slowly accelerate on the gas pedal. If the engine lifts to one side, your mount is broken.
Vrrrrrrroommm If the driver puts his foot down. Brrrr-clunk-rrrrr-clunk-rrrrrrrrr With added gear changes.
It shouldn't ...
A centipede with a wooden leg
If the clunk is most noticeable at takeoff or when coming to a stop, the u-joints on the drive shaft probably need to be replaced.
You may have U-Joint problems
The cast of Clunk-Click - 1973 includes: Gary Glitter as himself Jimmy Savile as Himself - Host Chicory Tip as Themselves