System mate 2000 boiling noise

Hello, We have a glow worm 18hxi and a system mate 2000 with switch. About two weeks ago we had a fault on the glow worm, a drip of water above the glow worm caused the thermistor to fail. We used the switch function to heat hot water until the repair guy changed the thermistor. Once this was changed everything worked fine, however that night the system mate made a very loud boiling type noise for a couple of mins every hour. We called the engineer out again and he said that it was air in the system and he let some air out from the system mate and the rest would dissipate over time. The noise doesn't happen as often now but two weeks on it is still making this noise at random times, waking us during the night. Very annoying. We have tried letting more air out of the system mate but it's not making any difference. I hear it at least 4 times a day and it's very disconcerting. Does anyone have any idea what is causing this. The heating and hot water are working perfectly.

Just thought I'd write an update. Noise wasn't caused by air. The issue turned out to be with the control board behind the back up switch.