Could use a filter sock if the noise is from bottom of tank if the noise is from the top of the tank you could possibly fill the tank up a little more.. so the water is constantly flowing.. cut the pipe down.. or even put a stronger return pump on to put more water in the top tank..
Most end the drain about an inch or so below the waterline when the system is running. With it 4-5 inches it make it harder for the system to purge all the air from the drain lines.