Update: It WAS a vacuum leak. I have a somewhat fabricated vacuum line arrangement because of the Explorer intake, and one of my aftermarket rubber elbows must have begun to deteriorate from exposure to the oil/fuel/combustion gasses (it was on the PCV line). It was hard to find, because the elbow looked perfectly healthy, but it had begun to swell and was not positively sealing on the metal barb fitting.
Anyway, I replaced it with fuel hose, buttoned everything back up, and now the noise is gone. I knew it had to be something simple!
Anyway, I replaced it with fuel hose, buttoned everything back up, and now the noise is gone. I knew it had to be something simple!