Every once in a while, my EKO 25 will get into this mode on startup where it does a low frequency pulsation - vibrates the boiler and the chimney. Can feel it through the house. I think it correlates with too much small wood that's too dry, but I'm not sure. Often when it does this, it will also have mini-explosions in the upper chamber that blow smoke backwards through the fan and slam the metal one-way fan damper shut. Eventually it will settle down and operate correctly, but it's annoying and hard to stop. Observation of the gasification chamber leads me to say that secondary combustion is not complete when this is happening. Flue temperatures also run higher - 300 - 325 vs 250 normally. Anyone else seen this or have any insight on the dynamics of what's actually happening inside?