[EricH]

I'd be inclined to say you're tripping breakers. 4WD long robots tend to either bounce like crazy or trip breakers when turning if they have medium/high traction wheels; this one is a breaker-tripper. And breakers tripping have a distinct clicking sound.

Another pair of motors in the drive, properly geared to match the existing CIMs, would help the breaker tripping by splitting the mechanical load between two motors on each side (and electrical load, too). Also, you may need a different ratio in the drivetrain; say more like a 3:1 or 4:1 than a 2:1.