I doubt there is water in the air duct. The joints are not sealed so it would be dripping on the floor. The sound may just be air in the heater core. As the engine rpms increase the water flowing through the coils might make a rushing sound. Check the coolant level in the overflow tank. Try setting the heat temp to max to increase the flow through the core. This may flush out any air or at least cause a change in the sound to verify this theory.

Another theory less likely is water in the door or rocker panels. Check the drains at the inside bottom corners of the doors and in the seam along the bottom edge of the body. What condition is the outside window scraper rubber?