Can anyone identify the cause of the following : Intermittently hearing what seems to be a metallic pipe vibration , after using & turning off hot water at kitchen sink . The vibration /murmur is for probably less than a second , can happen 1 -4 times & can occur between 5 & 20 mins after turning off hot tap . No vibration heard whilst water running . Hot water is fed by combi boiler in kitchen . Hot pipes serving kitchen sink & bathroom sink & cold mains pipe and pipe to water tank in loft all on the surface , no visible issues , no broken clips . Due to intermittent & random nature still not been able to identify which pipe is vibrating , I do not believe it is the plastic waste pipe . Note : I have got the mains flow turned down at the stop cock in the kitchen as the mains pressure / flow seems so fierce . I have also sent email to boiler manufacturer , seeking advice - do not appear to experience issue after running cold tap . Have been told it cold be air in the system ?? I have no plumbing knowledge , as you can tell . It seems to me that energy is being built up in a pipe which is then released by the vibration . Like I say it is intermittent , for example I have used two bowls of hot water this morning with no vibrations heard ? Thanks in advance for anyone reading this post & giving a response .