Our home was built in 1950. It is on a slab, and the returns are in the floor, run through concrete ducts. Where the returns come together under the furnace, there is an about 8" drop. While my husband was putting a new filter in, (under the furnace) he noticed that this area is filled with water, but only about an inch or two. He used a wetvac to suck the water out, and slowly but surely, the water returned. It does not reach the return ducts, and they are dry, however, in looking at this area below the furnace more closely, this has been happening for a while, because there is a line where the water has been and never rises above. I guess that explains why in the past when the furnace turns on it smells musty. This smell is bad today though. Any ideas?