be the pads in the front drivers discs not seated properly.
Was driving me nuts, I checked the wishbone, tie rod and ends, rear engine mount, gear linkages, etc etc and turns out to be the pads in the carriers not seated properly.
The fault was when you reverse brake, CLUNK. But if you reverse, slip into 1st gear power and go no clunk ?
So everytime the brake is touched when reversing you get the clunk. Stripper out the front pads and seated them properly and no more clunk.
Not sure how this happened as I did the front pads and discs my self (mk4 golf), veyr simple, not my first time either. Glad its sorted as was driving me nuts!!