I have just had my 20 year old cold water tank ball valve replaced because it was not closing properly and the tank was overfilling.
The new valve however is very noisey ie a loud hissing for a couple of minutes just before the valve closes completly. I never noticed this noise from my old valve. The young plumber who replaced the valve agrees that something is wrong and says that he has done the same job in other houses and had the same problem he does not know what the solution is.
I am wondering whether he has installed a low pressure valve instead of a high pressure one.
this may be of no use,but i had the same problem after having central heating installed.it turned out to be an isolator valve just before the water entered the tank.i replaced the isolator and now the excessive noise has gone