timc_fla

D2466 is an ASTM designation for a PVC, schedule 40 pipe with solvent connected ends. They really should have used a threaded check valve for easier replacement. This application is most likely not an "air relief valve" but a "fluid check valve". What happens is the pool pump comes on and water is pushed up into and through your solar collectors. As this is happening the air is being pushed out of the solar collection system. When you turn the pump off, either manually or on a timer, the check valve allows air to enter the solar collectors and allows the water to "drain down". This is common and normal behavior. The sound you are hearing sounds like the valve is not seating correctly and is allowing some loss of water, as well as air. Perhaps a bug or debris has gotten into the valve either through the vent on the check valve or through the water from the pool or perhaps the spring and/or the neoprene seal inside the valve body has deteriorated. If you have a salt water system, salt crystals could have formed inside the valve preventing a tight seal. You could try squirting some water directly into the check valve from the vent in an attempt to dislodge any foreign material with the pump turned off and the system drained down and then checking the system with the pump turned on. If you still hear the gurgling sound, I would say to replace the check valve.