"It seems to occur at higher throttle (over 60-70%) rather than being from a higher rpm."
That's a pretty precise description of pinking.
On a good engine with appropriate fuel it should never occur.
Modern cars (don't know about trucks, but diesel engines don't pink) have mechanical and electronic gizmos to prevent pinking. But you've got a Chinese bike there, resleeved and rebuilt. A bit of new cylinderhead gasket hanging inside the combustion chamber could cause it to pink along with dozens of other possibilities. (Is your spark plug of good quality?)
Most causes of pinking aren't particularly serious or damaging, but the pinking itself can be.
(Don't want to be contentious here, but there's a good chance, if you're under a certain age, you've never heard pinking. Once you have, you'll find it absolutely unmistakable next time)
When you hear it, change down a gear then open the throttle to the same 60-70%. If the sound has gone, then you've found the problem.
Retarding the ignition a tad will fix or reduce it. Maybe it wasn't set correctly after the rebuild.
As far as I know Chinese bikes are copies of older Japanese, so they probably have mechanical contact breakers.
Best thing is to go to a motorcycle shop and ask them to show you how to do it, or explain the problem and ask them to do it. Just 2 or 3 degrees, or they may already know the problem and know exactly how much adjustment is needed.
If, on the other hand, after changing down a gear the noise is the same, or worse, then you've got something else wrong, probably requiring as a first step a look inside the cylinder head and lifting the barrel. Maybe an after effect of the previous rebuild.
One other thing to consider, is there something loose and rattling? After the rebuild, has the exhaust been bolted back properly, and any heat guards that might be bolted onto the exhaust? The silencer?
All the engine bolts secure?
The horn bolted back on properly? All that sort of stuff, that can sound like the engine rattling but often isn't.