It’s an electric heat, forced air system. I’ve been noticing a peculiar smell in one room when the heat’s on. Sort of an irony - sulfur smell. Maybe the heating element is burning out? But I don’t smell it at any of the other duct outlets, leading me to think something might be in the duct. But it’s such a subtle odor - not as if something has crawled in there and died. Is my best bet to get one of those duct cleaner services? Where do I start trying to diagnose this thing? Any ideas on what it might be?