The gearboxes for both hummers and peg perego gators are under the back seat.
If you hear a clicking noise It is definatly the gearbox on the gator. The hummer gearboxes sometimes come loose from the motors, so it may be as simple as tightning the motor gearbox.
Here is how you take the gearboxes out from the gator.
1. disconnect battery
2. take hinge pins out from the dump bed to access the green cover plate.
3. take a flathead screwdriver and pry around the plastic cover to expose the gearboxes.
4. take the rear wheels off and remove the rear axle.
remove the motor gearboxes (they are interchangeable)
5. install the new gearboxes making sure the wires are secure and the wheels are going in the correct direction.
6. put back together.
7. hook up battery and your good to go!
If you have any problems are questions you can contact these guys at http://www.shopjandl.com/kids---baby-ride-on-toys-peg-perego-switches-and-shifters.html
They have a toll-free number and are very smart when it comes to these peg perego ride on toys.