helping my brother with his chevy its has a NP203 with the part time kit. The 4x4 works but makes a loud popping noise which seems to come from the t-case. If you leave the hubs unlocked and have it in 4x4 it doesn't do it or if you leave the hubs locked but are in 2 wheel drive it doesn't do it. It seems to only happen when there is a loud on it, like driving. We pulled the case out and everything seems tight and the chain is good. Anyone have any ideas what to look for?