mmm sounds like your have the same problem as me... please let me know how you go.
is your overflow empty or full? the aim is to get it full to the same level as the tank. My friend has it and your can't even hear anything. mine is empty and it does sound like a toilet bowl is leaking and is very enoying, please keep us posted on how you go and if you have fixed it.
this is what mine looks like. is your the same?