Boiler is a Viessmann combi. After the CH has been on, and the boiler shuts off, - the fan and pump (?) continue to overrun for a couple of minutes as expected, but when they shut off, there's a loud hum (which varies in pitch) for about 10 seconds or so as the boiler finally shuts off.
Also, like most combis, when there's been no heating demand for 24 hours, the pump (and fan?) runs for a few seconds and the buzzing/humming noise happens then as well. There are no noises when HW is being used.
I've searched old posts which point to a gas valve solenoid causing a loud hum however this wouldn't be activating at the 2 situations described above would it?
Also, like most combis, when there's been no heating demand for 24 hours, the pump (and fan?) runs for a few seconds and the buzzing/humming noise happens then as well. There are no noises when HW is being used.
I've searched old posts which point to a gas valve solenoid causing a loud hum however this wouldn't be activating at the 2 situations described above would it?