Over time the sides of the hopper on our Harman Accentra insert built up a coating of some sort. I'm trying to figure out if it is residue from the pellets we've used or ....? The slide plate is free and clean only the hopper walls seem to be involved. Scraping it off with a razor blade worked best, and it broke down almost like sawdust when scraped off. I started to clean before snapping the pictures so you can see the difference. Anyone have any idea if this is just baked on pellet remains or the sign of something wrong with the stove? Stove runs clean and hot with no apparent issues.