Oh man, I thought the new extractor-fan they fitted was going to be annoying because it basically never turned off.
Turns out that was just a high-humidity situation.
Also turns out that if the humidity is more or less exactly 70%, it keeps flipping on and off.
Each time with the Whir of the vent-closing/opening gate.
At random times, all day and all night.
Whir.
whir.
Whir.
Whir.
Oh well. I thought maybe it was broken and just stuck on. At least it's not broken. Yet....