Some info from here might help:
http://www.beananimal.com/projects/s...ow-system.aspx
A standpipe can only run silently under two conditions. 1) The volume of flow is low enough to allow the water to cascade down the walls of the standpipe, leaving an undisturbed column of air in the middle. When water falls vertically down a pipe, the water will adhere to the pipe's walls until the pipe's cross sectional area is about 1/4 full. At any higher volume the flow starts to become turbulent and traps air. The trapped air creates slurping and gurgling noises. 2) The volume of the flow is high enough to keep the pipe entrance submerged, creating a full siphon.