Well after all this time I have finally discovered the source! It's this:

I know what you're thinking; 'That's just a jar of purple liquid' Well you're quite right. It's methylated spirit, which is used to clean bass guitar strings.
For the first time ever I happened to be looking in the general direction of this jar, which lives on my windowsill, when the noise went off, and I saw the lid move! Presumably changing temperatures cause the metal to expand/contract until it reaches a point where the centre pops up or down (kind of like the test to see if a jar has been opened - the centre of the lid should be up if unopened.)
I'm well aware that nobody gives a shit about this apart from me, but I don't even care!
Today is a good day.