Not sure what you are doing ... perhaps it isn't an airlock ... I did discover in a DIY store a valve which will let air out of a pipe while not allowing the water to escape ... could you use one of these - they were rather expensive and designed to be fitted to radiators to rid of airlocks.
If it is noisy pipes that are the problem try supporting the pipe better - particularly at any bends.
Otherwise if you have an inverted U shape in your pipework (bad design) an air lock will be trapped despite a flow of water. Hope this helps.
Air in pipes is very suttle and the idea of water finding it's own level doesn't always seem to work - try this siphoning water with a hose which has a few inverted U shapes in its path.
Nyge, March 2011
