When I flush the toilet it makes a loud fog horn noise.
I have looked indside the cistern everything in there is plastic
I've noticed the noise has something to do with the ball and water filling the tank.
Please help

This noise is coming from the ballcock.
There are several things you could try;
1) Flush the toilet and when the noise starts go to the angle stop valve and start turning it clockwise until the noise stops. **OR**
2) shut the water off at the angle stop and remove the three screws on top of your ballcock. Pull the float ball and stem assembly up and pry out the washer, turn it over and reinstall. But before you reassemble place a pan over the ballcock and turn the water back on to flush out the supply.
Now reassemble and test,
Noise all gone now? Good luck, Tom