'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
