Top end noise?

Hi,
This is my first post and I could do with some help from you guys.
Iv just rebuilt my top end with a new piston kit and had my cylinder replated, it all went back together fine and starts first time, my problem is it's got a loud tapping noise from the top end now, iv checked the valve clearance twice and decompressor cable free play and it's all ok, iv got a sneaky suspicion it's got something to do with the decompression cam on the camshaft?
Any ideas or knowledge on this would be much appreciated?
Cheers

is the came and cam assembly stock?

Yeah everything is stock??