Question for knowledgeable lawn tractor mechanics (Sears Craftsman) -?

I "inherited" an older Sears Craftsman lawn tractor recently - it is a basic model, 11 hp, 4 speed, synchro balanced, 4 cycle. When I try cutting gr in say gears 1-4 with deck down & blades engaged, I am getting a loud noise from somewhere down it seems in the drive-train or maybe back (differential) area - It is like a loud "bang, bang" like a hammer hitting metal - Also when this occurs, the tractor seems to lurch or jump in & out of forward gear, like its losing the gear, going into neutral for a split second, then jumping (jerking) into gear again. It does it more and more as I am using it. I don't know anything about these tractors - I see there is a nylon drive belt running underneath from the engine (engine is in front of tractor) back to the top of the differential, where the belt goes around a "drive pulley" in the rear end. When parked, engine off and gears in Neutral, this drive belt is very very loose, lots of slack in it - not sure if this is normal in Neutral with engine off. Any suggestions on what I could do to degug and diagnose this problem (and fix it). Thanks very much !!!