geez. that made my skin crawl.

get yourself a long screw driver or spend the 5 bucks and get one of these.... Mechanic's Stethoscope
EDIT - 3 bucks

and try to figure out where that noise is coming from.
We can sit here and guess what it could be, but using one of those, you'll be able to tell if it is coming from the head, or the top of the block or the bottom and what cylinder it could be.

without knowing any of that,
my first thought was spun bearing/rod knock
then i was thinking maybe something dumb like a wrist pin.
I don't know that it sounds like valves, but it was interesting how the rattle got worse as the rpm's fell.

You could always get some thick oil or even some lucus, pop the valve cover off and pour it all over the cams/valves. quickly get the cover back on and start it back up. If the noise is coming from the valves, typically, pouring oil/lucus all over will at the very least, change the sound for a few moments.