I've dialed down the drain, I've slowed the return to 70%. And instead of gurgling toilet, I have a gerbil chewing on pvc noisies. Pinching the airline doesn't help. In the sump a few bubbles come in the water line in the over flow is a perfect 1/2" above the top of the bulkhead ring. NOTE: my overflow is not a bottom drain, it's a side drain. It's so close to the quietness of a full syphon...