I have a seperate gas fired DHW heater from my central heating. It supplies 3 showers, 2 baths, 7 sinks and a washing machine. The temperature is set at low end of the heat scale. A pressure vessel is also included un the system. For a little while now I have been experiencing a spluttering noise on one of the upstairs sinks which lasts for several seconds. This same problem is now showing itself on the kitchen sink. Is this some sort of air lock?