It may not be what others are talking about.. It might be the muffler vibrating at a very high hz.. Meaning.. very fast and short travel. Replicate the sound in park.. Take a towel or glove and place some force onto the muffler to "stabilize" it.. My truck seemed to start this sound at 2100rpm.. I had a friend hold it at that rpm.
Does it kinda sound like blowing across the top of an empty beer bottle? What I did was take 3 hose clamps and bent them to the size of the muffler.. Wrapped em around so that they hugged the muffler.. Wedged some small paper towel fragments between the spots where the adjustment screws for the clamps were.. No more weird bellowing echo. Kinda ghetto on paper but it worked out nicely.
The muffler is just too big.. It needs more structural stabilization esp in the middle.
:rofl: that's EXACTLY what it sounds like! I'll have to try that out and see if that helps.