There are two general approaches to the design of overflows: air dependent and full siphon. Durso, Stockman, etc. all require air and will fluctuate between a full siphon and air infused turbulent flow if they are air deprived resulting in the flushing sound you hear. A well tuned air-dependent system (i.e. Durso) will be very quite, but never silent. If your system flushes you need to allow more air into the top of the system. The more air you let in, however the noisier the system gets from the inrush of air and the flow of turbulent water. One approach to solve this is to just keep drilling holes until the flushing goes away and as along as the system continues to become quieter. After a point it will start making more air noise (not flushing) and you simply tape over a few holes and you're dialed in. Installing an adjustable air valve (e.g. John Guest fitting) makes dialing in easier in some, but not all, cases. Also, many have found that 1 1/4" pipe to the bulkhead is the minimum diameter for an effective and most quite Durso/Stockman system.
The outlet can be above or below the water level, but the air in the system must be allowed to escape and the further under water the outlet is located, the more back pressure there will be in the system and the more the air will build up in the the outlet line, resulting in increased burping in the sump. An outlet above water eliminates most burping, but typically results in more splashing and salt creep in the sump. Often the best location is just an inch or so underwater - this reduces splashing from the outlet and minimizes the burping. On occasion, I've found drilling a 1/4" hole in the side of the outlet just above the water level allows enough air out without splashing to reduce burping to a manageable level.
I, and many others, have moved away completely from the air dependent approach and have gone with a full siphon overflow. The main advantage is it is absolutely dead quite - zero noise, because there is no air in the system so the flow in the line is laminar and there is no burping in the sump because there is no air in the water. Several people have described full siphon systems and been given credit for various manifestations of the concept, BeanAnimal most recently, as well as others, but they are not new and have been around for a long time. They do require initial tuning which is accomplished with a ball valve, or preferably a gate valve, near the outlet to adjust the flow rate to be equal to the return pump volume. Full siphons can move more water than air dependent systems so they almost always have to be dialed back.
Both system really should have an emergency overflow which is just another stand pipe extending an inch or so above the desired tank water level.