We need more info from you because we can't see it from here. What type of air ducts? Where are they run, in the outside wall? From basement to second floor? First floor only? Where do you find this "standing water"? Do you have any exterior air ventilation intakes into your system? Anything will help. My only guess is that maybe your roof or siding leaks, and/or you have overflowing gutters and the ducts are in the exterior walls and the water seeps into them. This is just a guess.