Villeroy & Boch toilet (4 or 5 yrs old) cistern noise?

Hi. I have a V&B toilet with dual flush system. Over the last few days there is a continuous noise after flushing. It's like the cistern is finishing filling up but instead of stopping the noise carries on (it's not loud but it's noticeable). There is no sign of water leaking anywhere or that the bowl is filling when the noise occurs. What is it??!! Should I be worried?? Can I fix it easily? Last time I got a plumber out to one of these toilets he couldn't even work out how to lift the cistern lid, so I don't really fancy trying that route again! My understanding is that the cistern lid can be removed if the ring around the flush buttons is turned but I tried to turn this both ways and it didn't move much and I didn't want to force it. I then don't know what I'll find inside or what to do with it?! All help gratefully received:-)
Thanks.