Without wanting to start a battle here, I am extremely sensitive to sounds, spotting on high/low frequencies at low dB range, checked with Physicians, but maybe you're superior as you mention, so who knows 😂
I tried your exact same steps, not only pausing but muting as you said yesterday, and no, I didn't hear anything at all but dead silence, not even electronic sound/hum/buzz of any kind, so maybe you're more sensitive than me or both machines present same hiss.
Btw, as you mention "speakers don't make noise but the vents do", static air pressure changes from open ambient to a closed one, those vents can interfere cause they're in fact, small tunnels, so the hissing can be a perception from yours when you place your ears near them. You can accomplish a similar effect by sticking your hand as a roll (or a real toilet paper roll) to your ear, you'll experience how your ambient perception/hearing is different.
At this point, I don't know what to tell you regarding the machines, you started asking for temperatures and they were not concerning, the strange "speakers/sparkling" glitch appeared but didn't happen again, a technician reviewed the guts of that machine and told you nothing was wrong, despite that, you still were not confident, told me the static was still present and to perform those tests which in my case are negative; and yet you managed to get a new unit that has the same static noise problem plus bigger ones.
So my 2 cents, you should keep the one you think it's best or don't keep any at all, and don't settle till you're satisfied cause you clearly aren't, and IMHO, asking for other's thoughts on what bothers you can be counter-productive on some picky stuff. Honestly, follow your "big smart brain". All I can share is my experience.
Best of lucks