Can I suggest something that will probably be meaningless, but if it is your problem, it may save you a ton of time?
Is your drivebelt oversized or wearing out? That sounded a bit like mine when the tensioner was tapping against the stop because my belt was too long. Just look at the tensioner to tell. It should run between the two marks. The only difference I think I heard from your video was that when I heard that sound on my car it was not as precisely uniform as yours, and when I rev'ed up it went away as the motor torqued over and smoothed out from idle.
Again, probably meaningless, but I had to mention in case it saved you some time.