It is probably just Trying to rid itself of the effects of trolls that show up here from time to time.
Nothing to worry about, just a Troll fart. It has to stand still to do it.
It would be fun if you could get a sound byte of one on here.
I'd love to hear one of those.
Is there any change in exhaust pressure, cadence, etc. from the tail pipe, or is this noticed from the vicinity of the intake manifold, or purge valve?