"Random groaning from inlet valve in cistern?

'Groaning' as you call it is caused by the cystern ball cock valve slowly closing against the mains water which can be up to 60psi.
Do your self a favour and treat yourself to a TORBECK Valve. They work hydraulicaly and shut with a very quiet thump and stay shut without any of the screaming.
Cost less than a tenner and come in side or bottom entry. I have been using them for 20 years. For a fast result Google 'Torbeck Valve'

Geoff, deep down inside Wales
August 2012