John Deere 850 rear end noise

[JB4310]

I have a chance to buy an early 850 for small money ($1,800.) but it has a light knock coming from the rear end when ever the clutch is out, whether in gear or in neutral. (parked or driving) when the clutch is pushed in the noise goes away. The sound comes from right about under the seat.

The machine is in decent shape otherwise with 1600 hours, it's 2wd and 8 speed. was used exclusively for mowing with a woods 60 inch belly mower run off the rear pto.

Any ideas what I might be looking at for repair or future problems.
I only want the tractor for a back up, light chore machine and will remove the mower. I'm thinking that pto drive system might have something to do with the problem, since it has seen alot of use?

Thanks, John.