Confirmed from a shoot last night: it was interference from the boom operator's cell phone

* delurking *

A valuable lesson. In Europe, with GSM, mobile phones interfere with XLR cable runs with a very recognisable cadence.

If you'll all excuse the onomatopoeia, the 'bittly-bip bittly-bip bittly-bip buzz' is the standard call sign of a lonely cell phone as recorded by your sound equipment.

The bangle-jangle 'teaspoon in a glass' is so unlike this that for a while I was going for the analogue outcome, but its regularity and robotic stereo placement are interesting signs to look out for. Great to hear that it's been pinpointed.

Thanks to all for a very illuminating discussion. Lots learned!