Sounds like contactor tips.

One way to identify this is irrespective of whether its travelling in forward or reverse, make a note of the contactor that is chattering, then swap the motor cables off the top of the direction contactors with each other and try it. look to see if the same contactor is chattering. If the fault actually transfers across to the other contactor then good chance motor or cable fault, if the fault remains on that same contactor then high probability its the contactor most likely tips.