there should be an isolating valve on cold water inlet pipe,try flushing toilet and closing valve gradually until noise stops but the trade off is that your cistern will be slower to fill.I encountered this problem on a site I was working on and we ended up replacing ballcock,maybe the wrong orifice is fitted,if cistern is mains fed then white orifice should be used and if tank fed then red orifice should be fitted,do you have attic tanks or a combi system?If you have a combi boiler then it's likely that the cistern is mains fed.sometimes a flow restrictor helps(looks like a little plastic screw and is installed into ballcock at water connection)to check,shut of water supply to cistern,flush toilet(this relieves any pressure in the ballcock) undo connection and look for a restrictor,you can get high pressure(mains) and low pressure(tank) restrictors so these might be worth a go first.
steeveepee, November 2008
