Looking for some advice on a burning smell coming out of my vents. Smell comes through vents even with the engine off, and there is no smell running and driving with the blower turned off. The blower seems visually to be in fine condition, with no knocking sounds or huge amounts of dust. Tried taking it to a mechanic and he quoted me over $800 to fix an oil leak that could've been burning off onto the exhaust, but I know there are no major leaks and it doesn't make sense that the smell comes on even with the engine off. I live in a small town and don't have many options for a different mechanic, any ideas on other things to check/ try?