We have a carrier gas furnace. Starting about 2 months ago, VERY occasionally vents on one side of the house start emitting a very strong odor that I would describe as a smokey rubber odor. The smell happens at all times of the day, is expelled from the vents for about an hour, and within and hour or two it is gone. It seems very clearly to come from vents in one part of the house, and if you walk by the heater when it is coming out from the vents the heater does smell.

We have had the heater checked out, and he could find nothing wrong. The gas company has checked for gas and CO, and they found nothing. We've had the vents cleaned, and it still happens.