Hey guys...After reading through all the decriptions of the noise you're hearing, It kind of sounds to me like a small exhaust leak...especially the fact that it gets louder with increased throttle but not necessarily increased rpm. I've had header leaks on some of my cars in the past that were most noticible if you had the car in a semi lug and rolled the throttle on fairly agressively. As the cylinder pressure increased so did the noise.

Maybe something to look at...Check out your exhaust manifolds?...