Nah, I've pulled the final gear from the gear chain - it's not that. I've also pulled the belt from the motor and the motor still grinds under power. Removing the motor from the mount and it still feels aggressive under rotation, but it's the coupling of the motor to the mount that generates the awful noise. Perhaps some weird harmonics.

Apologies, btw, for the 80's music in the background. That's my sanity aid :)