im not familiar with this engine or hydrolic lifters really, btu if i was assuming that most of teh principles are the same as on a regular 8 valve n/a engine then...in my twisted opinion:
it is likely that any engine will have two tappets with play and two that wont budge by hand (depending on where the engine has stopped at. (the two 'slack' ones have their valves fully closed anfd the two that woond budge have the tappets pressed tightly against them since the valves will be partially open...may be one is opening and one is closing...so i wouldnt be suspicious of that at teh mo....
the noise in video 1 sounds to me like something is bumping against the rocker cover...try tapping the rocker cover with a blunt non metallic something or other and listen to teh 'timbre' of the sound and see if it is the same or very similar....I wonder if the cover has a very squashed oil seal and is fastened too far down as a result.....may be one of the tappet adjuster screws has worked very loose and is touching the cover??
just some thoughts
EDIT: (we all posted at the same time....lol) you might get sprayed with oil with the rocker cover offf.....hold the cover with two open hands and grip tightly...can you feel the knocking and does the sound dampen? maybe lift it SLIGHTLY and see if the knocking reduces...BE CAREFULL WITH YOUR FINGERS
