I had this problem and it turned out to be the diaphram in the ballcock valve. Where the ballcock presses against the 'button' at the valve unscrew it here (after turning the cold water off) and there is a small rubber disc, replaced this and the noise stopped. Come in a small pack fron DIY stores for a couple of quid at most.
Paul
July 2010
Automatic Answer
We may have found other answers which could help you.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix plumbing.