Kjh wrote in
roupdirect.com:

We have had a new Worcestershire Bosch 30i boiler. We have found
during the day when room stat is set to 18 deg no problem, at night
when we increase room stat to 22 deg there is an occasional humming
noise and it stops when we turn the hot tap on.


Flow resonance - try adjusting some lockshield valves to shange the flow.
Read up on "Balancing a Central Heating System" Could be a thermostative
valve being nearly closed and vibrating in the flow.