Mystery engine rattling noise?
 

Can you help me identify a mysterious rattling noise coming from the engine area of my car?
Rattling noise shows up when revs above about 2k.
Very 'thin?' sounding, pitch doesn't change with rpm.
I recorded a video to show the noise:


I noticed this rattling noise a while ago and have been unable to figure out the source.
It sounded a bit like the noise was coming from around the timing belty area, so I removed the cam cover, timing belt covers and harmonic balancer to try and see if something was loose in there (didn't find anything). I didn't take off the big main bold on the crank (21mm one) tocheck the woodruff key, so I don't know about that. (it is a 99, so the short nose problem is less likely, right?)
I've looked into what the sound of rod knock, detonation, TOB, etc and it doesnt quite sound the same.
I am hoping somebody here can help point me in the right direction.
The car runs great, other than the noise.