Hi Ralph, welcome aboard the tractor forum.
The first thing I would suspect is brakes dragging. Does the sound go away, or maybe get worse when you step on the brake pedal??
It's a Hydra-drive, no tranny, no gears to change. Hydraulic motor, driving a 2-speed (Hi-Low Range) differentinalIf you cannot duplicate the noise by trying one wheel at a time, block up the tractor with both wheels off the ground and run at idle. You can check by running thru the gears to see if the tranny is involved. You need to have a stethascope, or the old screwdriver in the ear to pinpoint where the noise is coming from, then decide on the next step.