herbie and basement sump

[MarkoD]

I just setup a new tank and the sump is in the basement.

I have a gate valve about 2 feet below the overflow, a union right below it and then about another 12 feet of 1.5 inch pvc down to the sump.

i cant seem to get the overflow completely silent.

it seems like there is air coming in through the union right below the gate valve which is causing noise. I have it tightened as much as possible but still i get noise from it.

any idea on how to resolve this without removing the union?