Is your crank pulley wobbling at all? Mine sounded just like that, and noticed the crank pulley (harmonic balancer) was wobbling a bit, which you can see your belt moving a bit where it comes off the crank pulley to the tensioner. Turns out they are a 2 piece design where there is rubber between the inner and outer ring pieces, and that had cracked and deteriorated. Got a Dorman unit, and was able to test out my Lisle Harmonic Balancer socket and HF high impact torque wrench on the crank pulley bolt, which worked quickly to get off the crank pulley bolt, compared to the manual method. You might want to get a flashlight on the crank pully as it is idling and see if you see it is wobbling/vibrating at all. If so that is likely your issue. Thought it was the tensioner adjuster, but that had already been replaced.Last night after a trip to KC, I swapped out the serpentine belt. I saw a small crack and threw in a Continental soon as we got home. I had hoped it would solve the odd noise I've had coming from over there. I'm attaching a video. Right around 18 seconds it is more prominent. I need to probably get a mechanics stethoscope like @plplplpl suggested. My screwdriver isn't getting down there far enough to check everything!