[Gabe001]

Update. It turns out it is mains borne interference. The culprit is something in the garden, as turning off this circuit stops the clicking/popping completely. Probably either the pond pump (350W), the filter UV lamp or the pond aerator, but could be one of the connections or the garden circuit breaker. Won't be too hard to pinpoint the culprit now. These will be off during the winter anyway but I'll investigate further shortly.

Thanks for the help.
Gabriel

Ps: I'd get rid of the fish, but the boss says no