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I wouldn't rule out the brakes or transmission, both are still turning when the car is rolling. In neutral, the only part of the transmission that slows down is the input shaft/intermediate shaft, but if the rear wheels are moving, so are all the gears.
If it's a dry scraping/rubbing sort of chuffing sound, something might be rubbing a tire or rotor, or the drive shaft. Approx how many "chuffs" per second at 20mph?
It might be as simple as a stick or brush caught under the car somewhere...
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