I have a heat pump with the blower in the attic. Recently the weather has been such that the unit will be in the heating mode during the night and the cooling mode during the day. I have noticed that the air coming out of the vents stinks when in cooling mode, but not heating mode. I assume the stink is because the trap is dried out and gungy air is being sucked in through the trap. Why would this not happen, or why would it not be noticeable, while in the heating mode?