I've just started the hunt for this and wondered if anyone had any suggestions. I took my bike in for the 6000km service and then headed out on a road trip (3000km). After a while I noticed this odd noise that I hadn't heard before.
Basically it goes like this. In first and second (when wind noise is low), when you let the RPM's come up in the mid range area you start to hear quite a bit of noise coming out the very front of the bike (not below you at the engine). It does go with the revs of the bike and it basically sounds like something is loose and vibrating. Once you shift and the RPM's drop, the sound goes away and it sounds normal again. I really don't remember it sounding like this before which is why I think it might be something loose. Now when this is happening, you don't feel anything different. The bike feels just the same as usual, just sounds really different.