the flushing noise means you probably have an overly large drain. If I open up the valve on my siphon line all the way open it can drain faster than the pump can return. So what happens is that it drains the weir down to the level of the standpipe. Then you have get the flushing noise constantly as the water is pulled in along with air.
You either need some sort of durso which prevents a full siphon and keeps the water moving in a thin sheet on the sides of the drain pipe with a column of air and provides a way for the air to escape. OR a herbie/bean animal set-up which uses a full siphon, which is quiet, but you have to have some sort of back up drain as balancing a fully siphon precisely with a return pump is essentially impossible.
