My zero turn Swisher does the same thing when the wheel pump belt starts to go loose. Make sure your belts are not worn enough to where they are running in the bottom of the V- Grooves on the pullies. Also check to see the pullies are not all glazed from belts that have slipped.
Mine eats belts left and right, so I always have spares on hand and go again. Other than that excellent machine.
I change oil in the hydro units from time to time.
Might want to check to see you are running the factory recommended fluid in your hydro units, can make a difference. That John Deere Hyguard Hydraulic Oil would probably work well in there.
There are 2 types of hydraulic fluids, one that will absorb a small amount of moisture and one that will allow moisture to settle to the bottom and can be drained out.
Good Luck
