Originally Posted by Jordo!

That sounds like a valve tap to me.

Spun rod bearings usually have a more hollow "tok tok tok" sound to them.

Could be mechanical (tight valve/bent valve/valves need adjusting -- which I don't think you can do on this head) or possibly related to a wonky VVEL sensor/controller.

Low oil levels could cause that too... but then where did the oil go?

Only other thing I can think of is what Steve was saying about the flex plate.

I'd get a leak down test and see what's up -- good luck!