High pitched whistle from pipework when flushing a toilet!?

Whenever the toilets in our house are flushed and the cisterns are refilling, the plumbing makes a loud high pitched whistle that really goes through you. I haven't got a clue what's causing it but if a tap is run while the cistern refills, this noise is less evident. Any suggestions as to what is causing this and what I need to do to fix it would be very welcome.

Dr Trevor Crumpton
May 2006