Whin from radiator stopped when hot tap is turned on

[Monty Dog]

Help I have a very load whin coming from a radiator which I can stop by turning on the hot tap? I'm not a plumper but I know the two systems are not joined so it makes no sense. I've had three plumbers looking at this but none have been able to fix it.
If I switch off the main stop cock we don't get a problem so I've assumed from this it must be the cold water pressure and the noise from the radiator is a deferred noise? So we've fitted a pressure release valve above the stop cock and a new stop cock.We've also re-pressurised the hot water tank but the noise is still the same. The noise is only stopped using one hot water tap,the first one in the house. It's the tap I thought so I fitted a new cassette in the tap. It's made no difference. If I turn this tap off from the mains I can then stop the noise using the next hot tap in the system and so on.
Every night we have to switch the stop cock off to get some sleep but during the day we have to put up with the load whining noise. Anyone got any ideas

 

[AlanE]

Try replacing the float valve washer in WC and any loft tank.

Have messaged mods as this thread should be in plumbing not Alarms, CCTV and Telephones.

 

[footprints]

Shot in the dark here, if the boiler is a combi the heating circuit is closed off when hot water is drawn. It might simply be the water passing through the rad valve causing noise and running hot changes the flow. Does turning off the rad help?