Does yours vibrate slightly through the floorpan of the car? If so my saph 4x4 does this as well. I've put mine down to the gearbox. As what it is exactly causing this i'm not sure. I have heard of a similar problem before.This other esc cos went back to a well known garage 3 or 4 times. Each time he got the car back it seemed worse. I think in the end he had a new gearbox fitted, but it was under warranty. Sorry i can't help you with this but will be interested to know what it is causing this rumble myself. Hope you get it sorted.

Cheers

Graham.