central heating divertor valve

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 overheating. The metal box is hot – is this normal?
Will the system have to be closed down and drained to replace the valve? If so waiting until the weather’s a bit warmer would be optimum.
Any advice would be very welcome, thanks in advance guys.
Cassie

Your diverter valve would normally feel hot when the system is running due to the hot water running through it. The humming may indicated that the motor is about to go but dont worry as your system doesnt normally need to be drained down to change it.

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 open but in view of the above I'm going to call the plumber. I rather have it replaced now than risk failure at an inconvenient time!
regards Cassie