Think maybe over thinking,pursume used to be ok,just replace ball valve washer,should correct siduation,the washer becomes hard and scaled up and does not open fully when ball float drops causing restriction and noise
as said sound stops when you turn bath tap on as you are also reducing water pressure at ball valve
Had one a the other month I changed when doing another job as making so much loud noise ,when customer came back and used thought I had broken toilet, it had been making so much noise ,she thought there was a macerator unit behind w/c, been like that for over a year since they moved in
Thought I may of got a tip but alas not to be
Mind you always say if you get a tip it means you have under charged so must not complain