A humming sound is often associated with an earth fault.
That it came and went fairly quickly, and hasn’t returned seems to imply something like a contact on the frame (MDF) which got knocked off again shortly after ... but it’s all just conjecture really.
It lasted a few days over a weekend and my ISP got onto the problem rapidly, an OR SFI was soon booked but I cancelled the visit due to the problem somehow being resolved remotely - if it had continued then perhaps I would have known the cause from the SFI. OR/wholesalers don't seem to have communicated back to my ISP regarding the issue. It remains a mystery that perhaps will never be known!
Thanks for your reply