I have a top level flat which is experiencing intermittent knocking in the mains pipe. I have been experiencing this issue for a couple of months now and am none the wiser as to the issue. It appears to be quite intermittent in, however there seems to be an increase in the frequency during the evening, between 8-10pm. It doesn't appear to be connected to anything in the flat as it seems to trigger even when no water has been run for a couple of hours. I did wonder whether it was possible that this was either the effects of another flats water hammer combining with a fault in our stop cock or air in the system. We did have a pressure arrestor fitted onto the boiler inlet however I don't think that has had an impact. If anyone has any suggestions that would be very useful before I try and start paying the call out charges for each attempted fix!