I can't listen to your engine from here, (Now in a hotel room in Gothenberg, Sweden and their firewalls are strict) but someone mentioned sewing machine. Is it all of your tappets or just some of them. If it were some of them, then the sound would be irregular at half engine speed. If all of them, then I am wondering whether you have a dodgy set of tappets or a worn cam shaft. Where did you buy the tappets? I got mine from GSF and they sound OK.
Next time you take the cam cover off, run a thumb nail over the cam lobes and check for any scoring. There is also a splash plate under the cam cover. Have you fitted that correctly? Or is the cam hitting it?
Turn the engine by hand until the dwell of the cam is at the bottom, (with the lobe sticking up) then see if you can get a feeler guage between the cam and the tappet. If the tappets are ok, there should not be a gap. (With engine warm) If there is no gap, then you need to start looking elsewhere for this noise. If there is a gap, then you still have a valve train issue.