My Jotul Castine is probably 6 years old. On some occasions this winter, I have noticed I can smell a hint of smoke after I shut the stove down for the evening. I have been able to sniff around to rule out my stove pipe connections. I use lots of high temp mortar to seal those joints every year. The smell seems to come from the back of the stove where the back panel meets the top of the stove. How do I fix this?