As I said before, the motor is vibrating when unmounted and has no load, so this is almost definitely a config issue. The machine doesn't actually rock about, however at certain speeds the vibration will catch it "in a sweet spot" and the whole thing sounds like I'm shaking a biscuit tin full of spanners. I'll have to re-check my wiring and try swapping to another drive, then another motor. Will also ask this question on the LinuxCNC forum as it may be a software setup issue...

And before you suggest using Mach, I'd like to point out that I've not used Windows since 2008. Yes, I'm a geek.