Well, it's hard to say without seeing th4e vehicle. But if you're saying that this noise only exists when the vehicle is in neutral, and it does it when the vehicle is sitting still, then it would seem to me that something in the tranny is not right.

1977 CJ-7, fiberglass body, AMC 360 w/ headers, DUI ignition, Edelbrock intake and Holley 4150 carb, TF999, Dana 300, 4.56 gears lockers, York air comp, 4" susp lift, 2" body lift, BFG 35" M-T tires, Megashifter, AGR pump & box, REP8000 winch.