Groaning Noise when water running

I came home to find Anglain Water had changed my water meter without warning while I was out, and put a card through the door saying that I may need to bleed the taps.
Despite bleeding all taps, I still sometimes get a loud groaning noise.
Usually, it's when no tap has been turned on for several hours (e.g. 1st thing in the morning) when a cold tap is turned on or a toilet flushed.
The noise stops instantly if a hot tap is then turned on (Boilermate 2 setup means hot water is cold main water run through Boilermate 2, of course)

Alternatively, if I deliberately run a hot tap first in the morning, it is OK and there isn't a noise when the cold tap is run or the toilet flushed.

Any ideas? Do I still have an airlock in the Boilermate?

Also, (may not be relevant) the new water meter makes a loud ticking noise when the water flow is high and Anglian Water engineer is coming back on Wednesday 25th April following my complaints.
Anything I should tell him about Boilermate?