Could be the cooling fan for the spindle motor.
If it's similar to mine (mine was a denford controller version), the spindle motor will have a forced cooling fan fitted, which I think runs whenever the machine is powered up and no e-stop/limit triggered.
And having just checked the wiring diagrams I've got, it is controlled by the main MDC contactor (I can't find the diagram, but I'm pretty sure that's powered via the E-stop relay), and has it's own dedicated circuit breaker labelled SMF. Flipping the SMF to off will soon tell you if that's the problem.