I have a customer complaining of a "sewer" smell coming from the ductwork. The smell only seems to be noticeable when the aux heat strip is on. The air handler is an older Trane unit with 10kw electric heat strip. There doesn't seem to be an odor when only Heat pump is engaged or when the fan is operated, only when the heat strip is on. The exterior of the ductwork seems to be OK when visually inspected. I haven't cut into it yet to check the interior. The odor seems more intense the longer the aux heat is on. Any ideas.