I just bought a 1980 GMC C3500 dump truck (work truck). The trans will shift throught the gears when the engine is off, but not when it's running. Where ever it is when you strat it up, that's where it stays, clutch in or out. If you put it in first, push the clutch in and start it, it lerches forward. If you start it in neutral and rev the engine, you can hear something rattling around in the bellhousing. I believe it is a SM465 (4 speed, granny low). 1. Any ideas as to what's wrong? 2. Any good tech articles for working on this trans? I've never worked on a manual before, anything would help. Thanks, Eric M.