Anyone with a 912s note a little bit of vibration from the engine in about the 2400 rpm range? Mine is pneumatically balanced right on and idles very smooth. Moving through 2400 or so, it has a 'rumble' to it. Then when you get beyond, it really smooths out. At 5000, I was amazed at how smooth it was. Put my hand on a carb and it is barely vibrating at all! Is this just perhaps a harmonic thingy going on? Not real worried. My Aeronca Chief rumbles and shakes at about the same spot (of course different RPM setting but still in the 1/3 throttle setting area).