[Al Skierkiewicz]

If both sides are identical and you are only tripping the breakers on one side, suspect binding of the mechanicals on that side. If that turns out to be OK, take a look at the wiring to the two motors. Even big wire has some resistance so if you have a lot of wire in the side that doesn't trip then the wire is limiting the current to that motor. It doesn't mean it's correct just that the resistance is keeping the current below trip. If you are using large diameter wheels, you may have to further reduce the speed to get the motor back into a lower current range. The clicking is definitely the circuit breaker. Remember these devices are temperature controlled and a hot breaker will trip at lower current than a cool one.