I have not had computer access for a few days so only just seen your helpful reply. Thanks very much trev7292. Yes, it was the metal motor box the noise was coming from and I am relieved that it can be changed without draining the system.
A week on it is 'so far so good' with the door left...
Is a loud hum from the divertor valve a sign of imminent failure? Seems to happen when the system has been on for several hours but only when the airing cupboard door is closed. If the door is left open the hum does not start – past two days anyway – so I’m guessing it is caused by...