Check your incomming water supply connection for a PRV Valve (Pressure limiting valve) as this could be the cause of your problem – as could a loose tap seat washer, a poorly fitted water pipe passing through a too large hole without support, a toilet cystern inlet valve seat a little sloppy.
A few ideas and thoughts for you to follow up on.
Selgas Services Ltd
Craftsman Gasfitters, Plumbers, Electrical Service Technicians