Recently had a new unvented cylinder installed (didn't go down the heatbank route in the end because I'm in a very hard water area, and because I wanted to get it all done quickly - i.e. by someone else).
It's been fine for a week, but now when I turn the hot tap on after no hot taps have been for a while, I get a few seconds of loud low pitched, periodic noise. Like an engine trying to start up. I can get a recording if it would help!
Any bets on what might cause this? I wondered if an airlock might do it?