Club Track day at Sandown

[Grasshoppr]

I was going through last prep for the club sprint day next weekend, bleeding brakes, changing rear pads etc... and the engine made a rattling noise??? :

I checked everything that I could including taking off the rocker cover with no luck as to what the problem is. out:

I thought it might be the new muffler tha I had installed but it is actually coming from the engine block and could be the cam or valve gear as it stops rattling when I bring up the revs :

So lets hope that the sickie I am going take this week will be enough time to get the green machine ready for Sunday :