Slurping/gurgling noises at the air line indicate a change in vacuum pressure at the venturi. This is usually caused by a partially blocked venturi or a partial blockage at the water inlet strainer.

Remove the pump from the skimmer and check that the venturi where the air inlet connects is clear. Somtimes it may be necessary to twist, by hand, an appropriate size drill bit through this to clear any deposits. Also check that the silicone pipe is free of blockages and blow through the orange tap in both directions whilst opening and closing it to remove any debris.

Using a tooth brush or simular, make sure the inlet strainer is clean and free of blockages before reassembling the unit and repriming it.