It may not matter at this point, but I just re-made the video. The first is longer than the original, the stethoscope was on the mic the whole time - so audio is improved, and you can now actually see where I am placing the probe. I sampled more spots too.
Video 1: at idle:
Link to original page on YouTube.
I also made a second video where I sampled the sound at the loudest point (carb), while revving the engine. I can almost swear I hear the noise stop when it revs to a point... leading me back to "slide rattle," but then again I was almost certain before that I heard it continue on the freeway, and before when revving it in my garage.... could this actually be intermittent when reved?
Video 2: revving
Link to original page on YouTube.