We just moved into a house that has an older (probably 20 years old) central AC unit. The unit cools the house very well. We have been noticing a very sour moldy smell coming from the central AC unit. We had an HVAC company come and service the unit and Freon levels were good and he cleaned the coils inside the attic unit. However, the AC still produces that sour smell from the vents. I went up to the attic and opened the top of the unit. I saw about an inch of water just sitting in the pan under the coils. I can't seem to get to where the water drains to see if it is clogged. It drains into the gutters near the roof on the 3rd floor. Also the drain connection to the unit is sealed and I can't open the PVC piping. I called up the HVAC guy and he said if it was not draining it would be spilling over and flowing everywhere and he said it's probably not an issue. He is probably right as it has been really hot and humid here in northern NJ and is probably producing lots of water. Though I am concerned about seeing the inch of water just sitting in the pan. Is this normal? Anyone have any ideas or suggestions? Also it is the sour smell that bothers us.