M542 driver - motor vibration at low speeds

[birchy]

I don't think your BOB is too close to the mains, I used CAT5 4core (2 x twisted pair) networking cable for the connections between the BOB and Stepper Drivers as this eliminates a lot of interference.
The problem you describe does not sound like a wiring problem and is likely to be resonance. I noticed the gap between the motor bracket and the machine was very small in places and I wondered if they were vibrating against each other like a tuning fork ?

I didn't think it was a wiring issue either, but wasn't sure if the steppers and drives are prone to doing weird things with mains cables in the vicinity, etc. Yes, the clearance are a bit tight in places but the motor is still resonating when I take it off the mounting and sit it on a rolled up rag. The noise is obviously less but the vibration can be clearly felt at certain low speeds.