Hey Scotty my brother has a 2001 gmc Sierra 1500 4wd auto transmission and when he is trying above 50mph It starts making a clacking noise like the gears in the transfer case are slapping. You don't feel anything really but its does shift a little rough. Any ideas on what it is? We do a lot of our own mechanic work but I'm not experienced with trans

those have REALLY weak transmission systems. Have a pro road test it and tell you, if he says tranny ir case is going out, get rid of it or spend a TON fixing it right.