No matter where the check valve is located it will still have a siphon effect.
I don't think that is correct. The siphon action only happens when the outlet is lower than reservoir of liquid. Without this, the horizontal pipe water will just drain out without the siphon action. His layout - siphon is happening from the high check valve to the outlet.
I believe your problem is a combination of errors. Your outlet is lower than your check valve, creating a mild siphon effect. Your "water head" above the check valve is minimal (8 inches). The combined
suckage/lower pressure) of the siphon and
little "water head" pressure above the check valve allows the
rising air bubbles in lower pipe (as it is draining out) to go past the valve flap and create your noise.
Possible solutions:
- remove the siphon action by following the advice above: open elbow at the end of the horizontal pipe with air gap, then into larger pipe or Reach4's air admittance valve.
- lower the check valve to "maybe" create enough water head pressure to overcome any rising air (and maybe eliminate siphon depending on grade levels?).
- don't use a check valve.
You will have to engage your engineer brain and evaluate using one or a combination of the above solutions
of your preference to remedy the check valve noise .