G'day everyone.
I have a slight issue with my 08 Impreza Hatchback Manual. Took it to the mechanic today he put it up on the hoist after a test drive and there is a thunking sound coming from the back of the gearbox near the first uni-joint. It gets faster the faster you go but going into neutral or clutch in or out doesn't affect the sound. Only the speed your going affects it. He said something may have binded up but can't be sure and doesn't want me to spend unnecessary money taking guesses. (anyway it could be viscous coupling?)
This all started when my car stopped dead reversing. Locked up all wheels but engine is fine.

Please help. Cheers!