This actually can be from the detergent and/or softener you use. Also, if any clothes have gotten caught on the back vent when drying and have melted somewhat on the vent/duct area. Have you noticed any damaged clothing? Softener has a tendency to turn almost black when it is burnt. This can happen when left over softener on clothes is mixed with too high of heating temps and cause that. You also might want to quit using fabric softener sheets. They will melt a residue unto the unit in various areas as well.