Where's this noise coming from?

Sounds like knocking due to too much timing advance to me, though piston slap or a bad lifter/rocker could sound like this too. Check the total advance on the timing and retard it anyway (even if it isn't too advanced) just to test the theory. If the sound is still present, take off the oil fill cap and the PVC valve - hold a small rubber hose in one ear and place the other end in the valve covers to see if you can detect the noise origin.

Good luck!