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Double check it's not a leaking exhaust header gasket first up.

Also - can't really tell from the video but do you have any of that Kleen system installed? You know the air intake into the front cam areas (for the exhaust valves). If so, maybe the reed valves in there are making noises?

I don't know - just trying to find something cheap for you to fix......

I had a good listen to your video...

It only happens when its warm, so that means it not something broken, or looses bouncing around in the engine, like bearings, or valve parts etc.

Its too fast to be caused by one valve, it could be loose exhaust gaskets, so check that carefully. Honestly in my opinion its chain noise, I would pull the valve cover and investigate.

That sound is not the normal "ticking" sound I was referring to earlier as it does not go away with increasing rpm's, since the sound it always there something is wrong, and it got to be in the chains....

Also if you pull the valve cover, warm up the engine first it may make easier to find your problem.

could it be the water pump impeller?

As a first step, I too would take off the valve cover and look for anything out of the ordinary. I agree with an earlier comment, the engine is not running evenly on all six cylinders. I wonder if a valve is not seating fully? That would account for both noise and uneven compression.
Good luck!