We reckon it might not be the C+S - as its doing it at idling. Which it wouldn't do if its the C+S.
What I believe it might actually be, is a vibration issue. The more I think about it the more it makes sense as well - its getting worse now as well so getting quite concerned about it.
It would account for it being worst at certain revs too. I believe it could be that one of the rubber engine mounts has perished and the engine is now vibrating against the frame or something. its booked into the garage for next week to check it out properly anyway now, as I still can't really be satisfied for definite as to what it is.
I recorded my ride home last night but it didn't sound right, the only solution I could come up with was to record from my phone whilst in my pocket - and the only noises I got were the phone moving about in my pocket