On Friday, 6 January 2017 21:53:03 UTC, Tim+ wrote:


Do you have a bypass valve that allows water to circulate back to the
boiler after the HW cylinder has reached its set temperature and the
motorised valve has closed?

I'm guessing that the noise relates to this.

Tim

--
Please don't feed the trolls

Yes, there is one, and I'm wondering if that's the issue too.

However it isn't my impression that the motorised valve has closed - but I will check this again to be certain.