grinding/popping noise

These are made by Traxxas. They're washers you can use to shim the diff ->
http://www2.towerhobbies.com/cgi-bin...?&I=LXDEA8&P=7


Though when I purchased a couple replacement ring gear parts from the LHS (http://www2.towerhobbies.com/cgi-bin...&I=LXSP75&P=ML) the teflon washer came in the "kit."

This takes some of the slop out of your diff and shoul yield better gear life.

Don't forget the diff grease and oil if you so desire.