Kegerator making odd noise

[Kyle]

I have a bar fridge (with a horizontal collar), a couple of taps on a passively cooled tower from a copper coil resting on the freezer and going up in the tower (see my avatar). I notice tonight it is making a cracking or loud tapping noise, once per 15 minutes or so. I am guessing I'm just overdue to defrost it, but has anyone experienced this before?

[Ian Grant]

Do you hear the noise at the start or end of the compressor cycle?

[Kyle]

It seems to be independent of the compressor cycle.

[Kyle]

Also, the timing is now completely random. Two cracking noises in succession, then nothing, then one, then nothing, in a random order