Thanks for the replies - you've given me some good stuff to think about, and I'll try various permutations to see if I can re-create the problem with other gear.

The suggestion that it's an artefact from the compressor is a good one - I use an Alesis 3630 for the mix bus compressor. But I don't hear this "air" when doodling around - only on the recording of doodling around. And it seems to randomly pop when the recording hits a transient - weird.

Anyway I'll test some stuff out and hopefully get to the bottom of it. Thanks for your help.