Loud humming in pipes when heating turns off

[davidc123]

I had to drain and refill the system a few weeks ago. Since then we're getting a loud humming noise coming through the pipework, but only when the heating turns off and the boiler goes into overrun for a few minutes. Have bled all rads repeatedly, no trapped air. Tried x200, didn't help.

Any idea what the cause could be? I can feel the pipe at the boiler vibrating but the fact that it happens when heating stops makes my think it's more likely to be a diverter valve than the boiler. If so does it need replacing or is there a simpler fix?

 

[cross thread]

Has it a by pass ? Have you altered it

 

[davidc123]

Had to Google it, might be wrong but i think it's the white knob in the photo. Seems like it bypasses the hot water valve, not sure if that's normal. But no it hasn't been altered.

What's the link between the bypass and vibrating pipes?