it's hard to hear the noise from your video, but if it comes from a slipping belt, you probably won't be able to hear it distinctly with a stethoscope / screwdriver, in this kind of case it could be difficult to identify the source this way.
If I were you, I would try to identify the nature of the noise (more like a whining slipping rubber or a whisthling failing bearing... As you said pulsating rubbing sound, it could be the belt slipping, but you are the most likely to know as you can hear it directly). I would also try to identify the source by simply listening to the engine bay (in case the stethoscope / screwdriver diagnostic didn't succeed as you expected), if it was possible, it could be hard to know from where really loud noises come from.
If you suspect that your belt could be slipping, you could check around the belt driven components for rubber dust and particles as a belt slipping around a pulley generally wear really fast, in this case you should find rubber all around the faulty component (assuming your engine has already run for an extended period of time under these conditions and your belt has worn enough). If you own an infrared thermometer, you could also measure the tempature of pulleys as the rubbing will increase temperature quickly.
You could measure the tension between the positive and negative terminals of your battery with the engine shut off, and then running, generally, you will hear that your battery should be around 12.5V when it's fully charged, and then the tension is supposed to raise by between 0.5V and 2V with the engine running, a low value could indicate that the alternator is not charging correctly, for example in case the belt is slipping around its pulley.
If you still cannot troubleshoot your issue, you could eventually try to remove one of the belts assuming it DOES NOT drive essential components like water pump (make sure it only drives A/C for instance), it could show you from which belt the noise comes from, or at least reduce the list of potentially failing components.
In case the cause of the noise is the belt slipping around the alternator, it does not always indicate that the alternator is failing, it could also be another electrical component that is failing. Before changing it at least check the battery.
Hope it helps.