Usually when a trans goes bad, it will start clicking, and the mower will hesitate with the clicking. I really think the trans belt is loose on your mower. That's what happens about 75% of the time. The idler pulley for the trans belt also goes bad. The newer version from JD is metal vs the plastic one Lesco supplied.
To check the belt, look under the unit, between the wheels, and you will see a big pulley and the belt around it. Grab the long side (opposite side where the idler pulley is) and pull on it. IF it gives more than a inch, in either tug direction, then the idler needs to be tightened.