The cold water inlet valve in the cistern is likely gone faulty. Sometimes, depending upon the design of the valve, you can remove and replace the internal washer but generally speaking it's usually worth just replacing the whole valve.
One thing you could try purely as a temporary solution until the washer or valve can be replaced, turn down the isolating valve to slow the water velocity down and it might stop the noise. Have a play with the valve open or closed to different amounts. Again, this is not really a proper solution but might stop you waking people up flushing the toilet at night until it can be properly sorted!
Can you upload a picture of the inlet valve inside the cistern please? It will help us decide what you need to do.
