I am wondering about an occasional loud clunking noise from my prefab chimney-stovepipe..I am a newbie and have been burning with this stove over a year now..All other noises seem normal i.e. when engaging combustor opening door to load more wood etc..the clunking noise will happen after stove has been burning for awhile..not sure if its when damper is nearly closed or its being caused by low oxygen or burst of air down chimney..any suggestions greatly appreciated..