Combi boiler system mystery noise?

ground floor flat, 2 above us. All use combi systems 5 yr old building. Neighbour upstairs came down today and asked if we kept hearing a noise which sounds like water rushing/filling something for 5-6 seconds then stops with a "thunk". I told him i'd been hearing it for about a year usually very early in the morn but just umed it was him turning on a tap/flushing loo etc. He thought it was us doing the same... Anyway nothing appears to be leaking and the noise isn't really loud (for us at least) but it is a mystery as I can't figure out in a combi system what would be making it. Is there some kind of pressure relief valve that would make such a sound? I usually notice it at about 5-6am but only cos it's very quiet at that time. The sound comes I think from somewhere in the ceiling in the hall just outside my bathroom, It seems to occur when the heating is off it may well happen when it's on but I can't be arsed to sit outside the bathroom waiting indefinitely for this phantom noise, however my neighbour has been off sick all week and I think it's making him crazy. For his sake please help, thanks in advance.