I was having the crunching noise after my build coming from the rear diff/rear slipper area. For 4 days sitting on the stand after putting in the electronics&I could not figure out what it was. It would make the sound as I accelerated on the trigger.
I took apart the whole rear of the car and took apart the diff. Everything seemed fine, so I put everything back 1 screw at a time, while running the motor connected to the slipper/diff, to see if the noise continued. After the whole process the noise went away&it sounded fine. I never found out what the noise was, but i figured it was binding of the rear transmission.?.? So i am keeping all the screws holding the rear diff/motor plate snug only/not tight!!
U definitely have to take the rear apart&rebuild it.

GL
THX