Sounds like lifter tap.
You probably have the original valve lash adjusters with the 1mm hole. There are a couple of things you can do to stop this annoyance.
1. Make sure your oil is clean and you are using the lowest weight for your climate and driving conditions. Thinner clean oil has a better chance of making it through the lifters.
2. Make sure you dont have some cheapy oil filter installed that doesnt have the anti-drain valve. This valve keeps the oil from draining away from the engine after shutdown so that it is present at startup. If you do the $14.95 quick oil changes, chances are you need to replace it with an OEM type oil filter.
3. Clean the valve lash adjusters.
4. Replace the valve lash adjusters with the improved version with the 3mm hole.
I had this annoying tapping and doing options 1 and 2 solved my problem.
I realized after I replied, that you did have a rebuild. It is possible that if enough material has been milled from the cylinder head or engine deck that the above options may not be enough to totally make the noise go away. You might have to perform the 2g head oil mod
to restore proper oil flow to the head.