"Thumping" sound on decel?

[DSG2003Mach1]

any thoughts on what to look at? Im not sure if its just a byproduct of tolerances in the aftermarket parts or what

Truetrac 31 spline diff
Moser 31 spline axles (with c-clips)
FRPP 4.10s
MT ET streets in case that somehow matters.
Also has Steed upper and lower control arms (might be transmitting more noise?)

on decel I'll get a rotational thumping type noise from the rear of the car. If I put moderate pressure on the brake pedal it will stop.

When originally chasing this I found that my rotor pros rotors had a little wobble to them (we'rent true). Those have been replaced, did not check the new ones before installing them. I also saw that the driver side rear rotor was very close to the caliper bracket, I spaced the bracket out with washers and now both sides the rotor appears to be pretty well centered in the caliper. Apparently thats not uncommon with aftermarket axles.