I have seen this years ago on one of my old Saabs, having all disk
brakes:
The mounting bracket for the brake assembly was loose and whenever I
changed from forward to going backward, I'd hear a 'clunk'. It was
intermittent at first but got progressively worse. I didn't find it
until much later until I took the brake assembly off and found it moved
back and forth -- it took a little force to make it move, but it
definitely moved. It unfortunately wore an elongated hole in the
mounting bracket, making the problem more pronounced as time went by.

I'd check the brake mounting. Make sure it is tight and torqued.
Hope that steers you in the right direction.
Regards,
Remco