Tornado fire in burn pot.

[ifixmy2]

My 25-pdvc has a good burn, but the flame rumbles randomly in a circular motion like a tornado.
It's just a slight noisy irritation at this point.
I burn somerset's exclusively for past 10yr. and moved stove outside back in the summer , to do a thorough clean out.
Any ideas what would be the cause?

 

[tlc1976]

Mine (different stove) does that often at startup. Looks neat. Never thought anything wrong, probably just the way the airflow bounces around vs the heat rising.