TL;DR - T3A singing is feature! Not a fault! I'm trying to understand your train of thought but this doesn't make sense to me if I look at the facts. Maybe I'm thinking about this in wrong way, not sure.
Let's assume my power supply is faulty - power supply is the source of this noise (or noise is coming from wall or whatever different source you can think of) - how this noise can get from station into handle? It would need to go through the cable, right? But in previous post you can see - there is nothing else than 24V heater power. Is my measurement wrong? It's possible, but we are talking about audible noise. It would be periodic repeating signal less than 100kHz. Such signal is easy to catch on oscilloscope - even on the shittiest oscilloscope with the worse probing setup. Thus I don't think my measurement is likely to be wrong. In such case - how could audible noise get from power supply into the handle if it doesn't go through the cable? It can't.
Then what is the source of this noise? Look carefully at the 24V wave form for the heater (first image in previous post). Heater is switched in audible frequency range. What if I don't hear any "noise"? Instead I hear the main 24V switching of the heater? Power for heater is 24Vpp 1 kHz square wave into 2.5 Ω heater winding -> 8A peak current. This output would make powerful audio amplifier. With such output I'm not surprised I'm hearing audible noise. I'm surprised that you don't hear it! T3A is very strong audio generator.
Sound is likely to be generated as the common "coil whine" - electromagnetically induced acoustic noise. Repeated current flow through conductor creates tiny movements of such conductor. Vibrating conductor touches surroundings and creates sound by tiny rubbing motion. If this repeating current flow has frequency in audible range then you will hear this as noise. Pumping 1 kHz @ 8A sure will make some movements somewhere. Perhaps this doesn't make movement in heater but maybe movement of contacts touching cartridge? Heater should be solidly potted thus no movement can happen? I can't really pin point where the sound is coming from - if it is coming from tip or handle.
Why did firmware upgrade change this? They likely changed switching frequency from less audible to more audible? I don't have any way to confirm this - but this makes the most sense.
Why you and others don't hear it? I have no idea. It would make sense if you have different hardware version of controller, and your version is using different switching frequency because of different mosfet or driver. This is possible but I don't think there are multiple revisions. In any case it would be awesome if you or anyone else (who has new firmware without noise) could check how switching wave form of the heater looks like. For the science!
I did open my unit when I got it - there were no major issues like bridged contacts with solder or damaged components from shipping. I also tested my internal power supply with electronic load and output was stable. No oscillation under different load conditions. I could replace internal PSU with external power supply but does it even make sense to do?
I can't confirm many things - thus I'm not sure if this is all correct. This is just what makes sense to me if I look at facts. It's not really important to me, but it's great mental exercise! I like T3A anyway, performance is solid and the bit of noise doesn't bother me. It's more curiosity than anything else.