Pop/bang at beginning of session

I have no idea where it comes from, but sometimes after I’ve been working on a session (as far as I can tell, after I’ve been soloing a lot of tracks for a while), I’ll un-solo and un-mute everything and there’s a consistent BANG at the beginning of my session that is not part of the recorded material. Quitting and re-starting Ardour makes it go away. What could it possibly be?

Sometimes it builds up if I play it repeatedly, sometimes not.

are you using any plugins? if so, one of them is almost certainly responsible.