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If it's coming from the rear, and not just out of you tailpipe, about the only thing it could be is oil buring off your exhuast. Just crawl under there and see if its oily.
Did you have greasy hands when you put on the new drums? If there is oil or grease on the outer surface of the drum, when I gets hot it will burn off, perhaps with some smoke. Does it smoke just when sitting there with the engine running, or after you have used the brakes enough to warm them up? Have you adjusted the Park brake so that it is not draging all the time?
New drums have a greasy film on them to keep them from rusting while in stock. They need to be cleaned with Brake Clean prior to installation. If not it will burn off so no worries.