Trapped air in toilet filling system

ferry

Each time the cistern refills after a flush we get a hum that sounds like trapped air. Has got progressively worse last couple of days.
Just does this with the downstairs cloakroom loo only.
Is there an easy way to bleed air from the system to stop this happening or might there be another reason for this?

Thanks as usual

southcoastrgi

change the washer in the ball valve/fill valve