Air in a water system will cause a jerky flow of water from a tap and make knocking noises. Usually, the air can be driven out by letting the tap run for a time. Very often the problem is in the hot tap in the kitchen. If this is so, it maybe possible to get rid of the airlock by fitting a hose between the hot and cold taps, opening the hot tap in the bathroom, and then opening both taps in the kitchen. The cold tap in the kitchen is usually on mains pressure, and this water under pressure will force the air to escape through the tap in the bathroom. If the kitchen tap is of the mixer type, then the connection to a hot tap will have to be elsewhere.
If there are several taps attached to one pipe, turn them all on, then turn them off in turn, starting from the one nearest the source. Air in a water system will rise. If the same pipe serves taps on different floors, run the lower tap until water flows without signs of air, then close it and do the same with the upper tap. If air becomes trapped in a high part of the system due to pipes being looped higher than the taps and none of these treatments will clear it, the fault will have to be attended to by a plumber.