My Enviro Mini has developed a random thump. When it goes into cool down mode the thump goes away. It seems to be related to the auger drive. The pellet level in the hopper seems to have no effect on the frequency or volume of the noise. The Mini was purchased new in 07-2012. This unit has not been abused, only clean, bagged pellets have been used and the auger has never jammed. I did have it seviced at the begining of this season. Any words of wisdom?