Motor paranoia. It's funny how whenever I hear a noise on my EMTB my first thought is "sounds like the motor". But, in reality, the worst noises have always been something else...loose cassette, mudguard catching the chain, chain suck on the ring, jockey wheels, wheel/freehub bearings, pawls etc. Also, sometimes the exact origin of a noise is difficult to pin down. What sounds like the crank is actually the back wheel or seatpost etc. The best way to solve problems like this is to start with the easiest bits to strip down/clean/grease/tighten, work through each of these, one at a time, making sure they're sorted and keep testing to see if that fixes the problem. In his case it seems to be something to do with the drivetrain or motor. So...check the cassette, freehub/pawls, wheel/hub bearings, jockey wheels, chain, chain ring & bolts, crankarms, pedals before making any conclusions about the motor.