We recently bought a 1960's house in Houston, TX. 2 stories, the 2nd story is half bedrooms and half attic. The main air handler and furnace is in the attic. After the AC has been running a while, a loud noise like a gurgling or knocking, somewhat randomly but about 1-3 seconds apart starts coming from the drain pipe in the mud room (where the washing machine drain hooks into the wall). It also feels like there's a little positive air pressure coming out of the drain pipe. Sometimes the AC can run for a while before this noise starts, but once it starts, it won't stop until the AC turns off, and then it stops instantly. The noise is loud enough that I can hear it in the living room over the TV so need to stop it. When the AC is on but before it starts gurgling, I can hear a little drip of water down the drain pipe but you have to be right next to the drain pipe to hear it.
There's a white PVC pipe running out of the AC which I assume is the condensate line. It runs in the direction of the drain pipe but because of the slope of the roof I can't access the area where they might meet. There is no trap in the condensate line that I can see. My theory is that condensation is building up in the line that runs to the drain pipe and after a while the air pressure and water are interacting, somehow sending vibrations into the drain pipe which is echoing out the mud room and through the house. Am I off base? Is the fix for this as simple as installing a trap at the condensate line?
Also, does anyone know what kind of ancient flex duct this is? Would new flex duct have better insulation and be more efficient?