My heating and air vents appear to be the same. When the unit (heat or air) is on there seems to be a vague odor which reminds me of cat litter. The heater is in the basement, the air unit outside. The filter is in the basement in a big metal cabinet. There is no odor in the basement. What can I do to get rid of the odor. I just moved in here in Feb. and at first thought the odor was from cats the previous may have had. But the only time it smells is when the unit is on.