Ah OK. I solved it.
I had the iZotope RX de-esser plugin automated so that it came on and then off again for a couple of seconds to reduce a particular bit of sibilance. The pop happened a couple of seconds after the plugin was deactivated again in the automation.
Seems strange that this artefact happens after the plugin has been deactivated, some kind of bug I guess. Anyway, I guess I will maybe just live with the sibilance, it's not as bad as the pop!
Ah, sorry, just worked this out. I had sustain left 'on' in the preceding MIDI region. Maybe I trimmed the region some time ago and didn't realise this happened. So many little things to trip you up!
thesheep's post in Intermittent audio pops in specific places, except when Audio Track Editor is open was marked as the answer