Odd noise

[Charles P]

i've got an unpleasant rattle on a trailing throttle in a freshly rebuilt blown engine.
The video is here (a bit rubbish but you should be able to hear the noise)
https://www.dropbox.com/s/sihsfa02cua9zc...4.MP4?dl=0

Things that I've checked:
The crank isn't moving 
Depressing the clutch doesn't alter it
The blower isn't making the noise - uncoupling the blower doesn't stop the noise
The cam isn't moving (checked by removing the dynamo and looking in)
It's not the starter motor touching the flywheel 

Suggestions welcome please before I start stripping it again.

Thanks

Charles