Very high pitched humming sound when we flush toilet, what can i do to stop it?.

sounds to me like washer in Float valve either replace diaphram washer of renew float valve

the noise usually is the water inlet (ballcock or similar) inside the toilet cistern, when the water comes in if it's high pressure in can cause the humming sound. The other is the ballcock in the cold water tank in the loft, as this can cause humming through the cold water system.

New ball valve and possibly a shock arrestor to stop water hammer

It sounds like water hammer, try turning main stop cock fully open if no good then change ball valve as not worth repairing as only cheap this should releave you of problem.