Strange noise from Water pipe

[efiste2]

Every so oftem there is a really LOUD noise coming from a water pipe in our house, I have narrowed it down to the cistern on the Wc. I can only describe the sound as though someone is jigsawing through a beam upstairs, its like a really load buzzing/ringing sound the echoes through the house, Im assuming its happening as the ballcock closes the water off to the cistern. as the noise gradually stops. Has anyone any ideas on how to solve it,

 

[seco services]

change the rubber diaphragm or complete valve

 

[efiste2]

This noise has become more regular, so i had a look and i have narrowed it down to the inlet/ballcock assembly valve. and this noise occurs when there is a slight amount of water flowing through it, and whilst the noise is occuring if i move the assembly (point B) slightly the noise stops. I have adjusted the screw (point A) so that the valve assembly no longer has as much play in it, and this seems to have cured it. Am i right in thinking its the valve opening and closing very very slightly casuing the inlet valve and connected water pipe to vibrate against a joist or similar. and also can you more learned chaps tell me what the red thing at (point C) does I apologise now if im utterly wrong