If the bathroom cold tap and cistern are fed from a cold water tank then one
possibility is that the washer/diaphragm in the ball valve is worn, and the
noise is being caused by water hammer. To confirm this, open the tap and/or
flush the toilet to cause the load hum, then push the round plastic float in
the cold water tank down to fully open the valve. If the hum stops then the
valve is the problem.
HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.