constant buzzing

[buzzatnight]

I have a Baxi Solo 2 50 PF - installed 1999
When the heating turns itself off there is a buzzing sound (sounds like the noise it used to make for a fraction of a second when coming on or going off) from the electrical part under the boiler. It started about 2 weeks ago and lasted only a few minutes getting longer each time the last 2 nights it hasn't stopped until the heating comes back on (if I turn the electricity supply off it stops but comes back on again when I reconnect)

 

[Whiter]

Check the switch live voltage, if buzzing occurs when switch live voltage is not at full potential, then you may need to check you motorised valve.

 

[keego]

There are a couple of relays on the board that can buzz if they're on the way out, my own boiler is doing it occasionally but I turn the power off briefly to stop it. I know I should replace the board but you know what it's like, you can never find anyone to do it.
It's like they say, never buy a car off a mechanic!