herbie and basement sump

[SeaHorse_Fanatic]

If its air entering and no water leaking out of the union, perhaps a very, very light coat of aquarium safe silicone can seal the leak and make it more air tight. Just enough so that you can still open up the union if you need to. [I'm assuming that opening the union is not something that you will normally need to do, other than for a major plumbing project or tank tear-down]. If nothing else, it should eliminate the leaking union as a possible cause and you can check else where in your plumbing for the source of the air/noise.