I just purchased and installed a used Lennox Traditions T300P pellet stove. I think it’s a great looking stove; it's in great condition and seems to work fine, except when heating, after the combustion air blower slows down, the stove makes a loud rumbling for 2-3 minutes. This is probably 8-9 minutes after the stove turns on. It’s not the combustion or convention blowers, they seem to run smoothly. It’s a loud, lower rumble sound that can be heard upstairs in the bedroom, with the stove on the main floor below. When I open the front door ever so slightly, the sound stops. If I close the door tight again, the sound returns. It only happens for a couple minutes during each heating cycle. I have played with the draft control and it doesn’t seem to make a difference all open, in the middle, or closed. I have cleaned the stove well and it does seem to help a little, though not eliminate the sound. When the stove goes a few more days between cleanings, the sound does seem to get louder. To me it seems like the stove is wanting for more air. Any ideas? Any thoughts would be helpful. Thanks. KL