bees?

gas meter?
water meter?
could still be electrical. Turn off you main breaker and see if it goes away.

Will try turning the breaker off and see if that works thanks - will let you know as it is really getting to me now

If that stops the noise, have someone relocate the transformer or remount it with some vibration isolation.
I suspect that the noise is because the transformer is mounted to your ductwork or the furnace enclosure and the vibration (which is normal) is resonating through the metal.

Electrician now ruled out it being any of the meters or the breaker after switching everything off and the buzzing still being present. It appears to be coming from between the ceiling of that cupboard and the floorboards of the landing above it. Could there be anything that would make this sound in that area, there may be some pipes but nothing electrical as the electric was switched off and the buzzing still sounded. Any ideas or do I refer to the plumbing section before tearing up the floorboards?

Did you turn the water main off at the street or the side of the house?
Any possibility that you have a nest of wasps, yellow jackets, or honey bees?

Tried turning gas & electricity off but not water will try that and get back to you. No bees or wasps as can now hear it in most parts of house obviously louder when the house is quiet. Thanks

Hi, what issue was discovered? As you can see, I'm up researching because I have the same issue. Ive tried shutting off the electricity too but the buzzing persists. Please advise.