SOLVED
I had this exact same issue, and a new clutch Genuine Honda clutch and Flywheel fixed it.
Tom at TGM said the clutch springs were causing the rattling. Got a flywheel for good measure as it was pretty knackered. Might be just the clutch that rectified the noise, or a combination of the two being replaced. Either way, no more noise and engine running a lot smoother.
Sent from my iPhone using Tapatalk