Is it a loud, sharp click that just happens occasionally when you rev the car? The kind of noise that makes you verry worried for a split second when you hear it? I think i know which sound you're on about, i have it on my 1.2 GS special. I've never been able to find the cause but i believe it is a mechanism on or within the carb, simply snapping shut a bit over enthusiastically. It has never caused any adverse effects or reliability issues for me, so i don't worry about it. If you have the same thing going on then i would advise you not to worry about it, provided that the engine is running normally.