Had a similar sound, which started aroun 3000rpm, then went lower to 2000rpm, and then extended from 2000rpm to 4000rpm. It was a rod bearing which was spinning on the crank and ended up as thin as a paper sheet.
http://www.flickr.com/photos/47779682@N05/4460248187/It was a knock, a heavy knock. Many mechanics suspected the piston bolt because the sound could be heard up in the head, somewhat like the piston was hitting the head. As I said, in the begining you could barely hear it, but after a while it got reallt loud.
Hope is not the case for you, but if it is take off the oil pan and do a quick check.
Good luck