central Air Conditioning
I'm having a bit of difficulty imagining what the issue and set up is like, if you do or do not have a trap installed, if the water is backing up from th drain line or if it's condensation dripping from the coil or what..?
Logic is saying that the drain line is backed up so the water is building up in the furnace, but you've said the drain line is clear. If it is a water backing up in the drain line and building up in the furnace area than I'd suspect it's a high humidity issue so an excess of moisture is being pulled from the air and now the lack of a p-trap, assuming you don't have one, is rearing it's ugly head. To properly drain it needs a trap. You can install one just beneath the home so the drain would drop down from the unit directly into the trap and then drain out to wherever the drain line terminates.
Can take some pics?