tappety noise after engine rebuild

All
just put my engine back together and all is well until I take it up to about 2500 rpm. At that speed the engine makes an itermittent rattling sound. It sounds like the tappets go out of adjustment. I have set them now to 10 thou about 6 times and it keeps happening. I don't think its the bottom end but will take the sump off tonight just to have a look at the crank and check the little ends.
The engine runs really sweet and at idle you can not hear anything.
I have replaced the rocker shaft but not the rockers or push rods. Could this be the problem?
Any ideas would be very helpful.

Cheers

When you have the sump off, check the BIG ends, they seem to be prone to wear. Probably because previous owners over the years don't/didn't change the oil often enough......If there is wear on them, the main bearings are most likely the same..

Thanks Mad git

Big ends, mains, little ends, pistons are all new. Crank has been reground and I am pretty confident that all is well down there. I'll have a look anyway. Also the noise is not constant or regular it comes and goes. Its winding me RIGHT up. Just spent bundles on the machining etc... and I don't want to do any damage.

crank regrinding can often be poor ,tappet noise is have the speed of crank noise

Yep, I'll have a look tonight. I think any issues will manifest themselves as scratches/ scoring.

Timing chain rattling? Is the adjuster spring on and the anti-chatter plates set correct?

Engine knock...recheck timing, does it go away if you retard the timing?

A length of 1"x1" used as a stethoscope can sometimes help but watch out for moving parts, it hurst when the stethoscope turns into a projectile if its stuck in your ear!

I thought of the chain too but it seems to be coming from the top of the engine. Chain is also new.