yep airlocked water in pipe blow hard up the cold tap if you have strong lungs. Or link the hot and cold taps in the sink together with short hose with tap connectors on hose ends hold them on two people make it easier you hold and end each.
turn on the taps together and the water from the hot tap should force out back to the tank the airlocked water. turn both taps off and release the cold tap hose and try it. Oh allow the hot water to go cold before you try it or you'll get burnt by hot water. that's how I've cleared airlocks.
BB
January 2010