Would it help isolate the problem source, if Martin would unplug each of the servo motor cables, one at a time?
A vibration might potentially be caused by a stuck servo motor, a fallen screw/part inside the motor box, or its rubbing internally against its case/mount. Perhaps a wonky servo shaft. Might also explain the electrical tingle he feels from touching the CP4, if the short was caused by a motor power attachment, mechanical failure, leading to a DC short to the case and mount. Otherwise, if there is still mount vibration when either motor’s power is isolated, then as you suggested, it may likely be a power source leak going to both motors via the CP4, causing the mount to conduct the vibration.
Pure conjecture on my part, but I would eliminate each motor’s power feed, in turn, since a “mechanical vibration” can only emanate from a mechanical source and there are just these two, on the mount. Besides changing the DC power supply, I suggest completely disconnecting all accessories on the telescope – just power the mount directly, in case a current leak is coming to the CP4 via some other device/accessory.