This question may be better suited to the physics stack exchange but I figure I’ll ask here as well. So a month or two ago after trying to learn to overtone sing for a couple days I finally managed to produce perceptible overtones over a fundamental. In the following week I practiced my new skill pretty obsessively. A side effect I’ve found is that now, with some effort, I am able to hear the overtones in other instruments. For example, if I sustain a middle C on my upright piano, with conscious effort I can hear the first six partials (fundamental (C4), C5, G5, C6, E6, G6). I can barely hear the sixth partial and cannot hear any trace of the seventh or above. However, the partials do not sound like sustained pitches that slowly attenuate like I would expect. Instead there is a distinct pulsating effect where, for any partial save the first, the volume oscillates between between audible and inaudible at a frequency somewhere in the range of .1Hz–10Hz. What causes this effect? It seems similar to the dissonant beating produced by two sine waves that are slightly out of tune with each other. Is this what is happening i.e. the different C-partials, E-partials etc. are slightly out of tune with each other?
Here is a recording I took: https://drive.google.com/file/d/1ulnHyYwEmGaxgT-hB5XxpaeFVwTkTWDF/view?usp=drivesdk The oscillations appear to be around 0.5 Hz