You should have the main drain a few inches below the sump water level. This will eliminate and noise from the main drain into the sump. The emergency should be a inch or so above the sump water level. Once tuned correctly there will be a little trickle noise from the emergency and no noise from the main. The reason I keep the emergency above the water level of the sump is so that if there is a problem with the main and there is a lot of flow through the emergency it will make noise so that I know there is an issue with the main