It sounds like a cable or braclet dragging on the boom pole to me. That or a bearing in a fan that is not behaving. If the boom ops heart is going, that pulse could be doing cover the consistancy. Juat another theory.

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

The moral is to make sure anyone near the sound equipment, the boom operator foremost, has their cell phone turned off.

So, just before every shoot, call him. If he answers.... ;)

Sheesh, you'd think of all people a sound guy would know better!

The second moral is: you get what you pay for. The sound guy was just my friend.

* 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!