The rear tyre rubbing on the hugger was my first suspicion. The fact that I'd recently switched from PR2s to PR4s, and your post above, nickodemon, got me thinking that it sounds quite possible that this is what it could be. Quite likely I first started to hear it shortly after that switch, mileage covered since would be about right. Looking around it in situ, it didn't look like anything was close to the tyre (or chain for that matter), but I did continue to think maybe it was just somewhere I couldn't see properly. This morning, I took the hugger off. Couldn't see any obvious places where anything might be coming into contact with it, so took the bike out for a test spin. Well, it's definitely loads better. But, I swear I could still hear that same noise on a couple of occasions at low revs, just when accelerating gently in traffic. Tried different revs in different gears, and it was difficult to reproduce, but I'm nearly convinced it's still there a bit, although much, much less loud.
So now I'm beginning to wonder if maybe fazer-rat is right, and it could be a normal thing for these bikes, maybe some worse than others? Fact is, once I go over certain revs, I can't now hear it, with the hugger off, because the can just drowns it out, if indeed it is still there. So I'm wondering, could it be that it was there all along and the hugger actually amplified it? But hang on, when I first had the bike, I never noticed it. It's definitely something that has started in the last few thousand miles, or at least, has got to the point where I could clearly hear it in that time. I must've covered at least 15k miles on this bike before the noise first manifested itself. And it had got to the point where it was bloody loud!
Well, the test ride today was only short, 7 or 8 miles, so maybe inconclusive, need to take it out for a longer spin, get everything properly warmed up.