faulty valve stem, loose washer, toilet valve, failed solenoid valve in dishwasher or clothes washer,failed prv (pressure reg valve), partially plugged prv, most models have a stainless screen on the inlet side.
,, or air lock can also cause this... does one fixture alone set it off? does opening another valve in the house stop it? yes, i get alot of calls for this,, it can be a pain in the arse to find... my moms place (on well) creates an air lock,, every couple months i have to shut down the main, drain the house at the lowest point and then refill the system slowly with all the faucets open, closing them starting at the lowest. let us know how we can fix it over the interweb!!! cuz travel time and ferry costs still would not be worth a visit from me.