BBS02 conversion clunk/thunk when motor shuts off?

[Bralexander]

Hello everyone, I did a BBS02 conversion (with 42t bling ring) recently, on my Polygon Heist X2, which is basically new with 200ish miles on it. I've done some programming on the BBS02, in accordance with the Penoff settings, which works great for me, and my style of riding, but I have one problem. I get a clunk/thunk when the motor stops, or disengages? The start is nice and smooth, ramps up and continues power just fine, but as soon as I let off throttle or pas, CLUNK! I am not sure if it is the motor or maybe the hub/freehub, or even the cassette. It seems like it shouldn't be any of those, since they are all basically new, but I can't seem to figure out how to isolate what is causing the problem. Hopefully someone can help. Thanks!

 

[Hwy89]

Most likely the motor is rotating slightly in the bottom bracket and clunking against the frame.

 

[Bralexander]

I have tightened the motor as much as possible (even using a few taps with a mallet, after tightening as much as possible by hand) and there is a spacer between the motor and the downtube that hasn't moved even slighty, so I can't see how the motor could be rotating. I also just tried to move the motor by hand, to check for play, and there is absolutely no movement, even while using a considerable amount of my own bodyweight.