Late to the party, been busy.
Anywho, my 390 had a hellacious rattle/knock at idle. I got all nutted up thinking possibly my crank was going bad. But then I had the idea to just try a new Meteor piston as I would be getting one for a complete overhaul anyway. End of the day, the rattle went almost completely away with the new piston.
Haven't even put my 390 in wood yet though, been busy plowing snow.