[Lord of Hog]

If it seems to be coming from somewhere behind your central vents then I seem to recall it's sticking flaps. Mine used to do it regularly but then stopped. If it's any help it seemed to do it a lot less if I didn't have the temperature set to the same on each side. You can get at the flaps and lubricate the spindles but don't overdo it or you'll never get rid of the smell.