Repair a V&B dual flush toilet?

I have the same issue with my V&B toilet - continues to make a noise like it's filling up with water - no leakage though - I can get the toilet lid off but I can't fix the noise - .... same issue as below....

i. 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??