u/AlternativeFine6636

I've noticed a bit louder buzzing coming out of the circled area (contractors?). I switched off the the power to all the heaters and turned it back on one by one. It appears that just one heater is triggering the buzzing. It is a storage heater at overnight tarrifs, but hasn't been in use last 3 days - the buzzing stopped when the overnight timer switches off. What could be the reason of the buzzing? Does that mean that even though the heater is off it still has power pumped into it overnight? After switching off the power supply to the heater for 30 seconds and turning it back on, the buzzing is so soft now you can barrely hear it.

p.s: the circled area is warm to touch, but not hot.