Herbi overflow making noise

[fkshiu]

Your main drain standpipe (the one on the left in the pic) is too tall as someone mentioned. A Herbie is completely gravity driven so there must be a certain amount of water on top of the main drain for it to work.

The water going down the main drain now is causing a vortex and sucking down air as well. That's what is causing your noise. So cut down that pipe until your not getting air sucked down that pipe.

Also, I hope that you're using a gate valve to control flow instead of a ball valve. Lastly, ensure that the exit of the main drain is completely submerged under water.