I quickly flipped into techie-diagnosis mode and started examining the aircon, fan, proposed that it might be the roof box noise coming through automatic cold air vents on the front etc.
It perplexed us (and at one stage we caught some taxi talk too) for the next half hour, with nic saying it was coming from her left, and me saying it was on my right, and it sounded like the sort of "flat noise" you get which is often hard to place.
Nic then suggested it sounded like static on a radio, at which point I remembered I had a walkie-talkie in my pocket. It had switched itself on against the centre console!
A sobering revelation. The WT was in my pocket as it's the orphan of the one I lost: I'm hoping to find another pair to go with it, and had been looking earlier in the day.
Sometimes though, occam's razor really ought to be applied early. Consider my forehead slapped. :)