It blows my mind that in 2020 people think their computer should be silent. Can it be nearly silent on some tasks? Sure, but most people are surfing the web (which thanks to all the crap added to web pages, can be taxing depending on the page), or use Youtube (video is taxing). Computers need to keep themselves cool when they perform tasks that will cause them to work harder and to do this the fans kick on. Without them your chip will heat up to the point it'll shut itself off, but before it does that all your stuffwill stutter and lag.
Expecting a computer to be silent is just silly and unrealistic.
Nonsense. You're exactly wrong. Modern CPUs use an astonishingly small amount of power when they're not being taxed. (And no, web browsing and playing YouTube videos is not taxing to any computer made in the last 10 years. Check Activity Monitor.)
Apple made the 2015 MacBook which was completely passively cooled with a teensy little heatsink. People can use that computer for basic tasks all day long and it works great. Web browsing, YouTube, etc. etc. Sure, if you max out the CPU for 30+ seconds, it will start to throttle. Meaning that it will run somewhat slower, but it will still be perfectly usable. It will not "shut itself off" as you say.
My 2015 MacBook Pro is semi-passively cooled. The fan is literally stopped almost the entire time I'm using it to do anything. I have to run one of the CPU cores at 100% for about 15 seconds before the fan even turns on. This happens to me very rarely.
So it is absolutely 100% possible for Apple to make a computer that's literally silent, since they've already done so. And it's 100% possible for them to make a computer that's usually silent, i.e., semi-passively cooled, since they've already done so. So I don't think it's unreasonable at all to expect all their computers to be silent when doing light-duty work.
The logic board in a 2015 MacBook is about the size of an iPhone 6, and only a couple millimeters thick. Imagine that as the basis of a hypothetical silent iMac. To drive the 5K display you'd probably want a discrete GPU, though, so maybe double the size of the hypothetical logic board. Add a nice heatsink (really, anything would be nice compared to a 2015 MacBook's heatsink) and some vertically-oriented vents and it would probably never throttle. The internal power supply could be much smaller since it wouldn't have to support a desktop CPU. The whole thing could be the size of a typical 27" monitor that you might see at Best Buy, i.e., it wouldn't need the huge bezels that current iMacs have.
iMac Air, anyone?
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Even those aren't truly silent though. Memory, processors, and SSDs can all make noise. My 2018 MBP makes noise whenever I move the mouse when it's powering my LG 5K. It's not loud, but I almost wish it were. It's just loud enough to make me wonder if I was imagining things.
Well, alternating current will generate a magnetic field which can cause nearby metal things to vibrate. The things themselves might make a noise because they're vibrating, or they might vibrate against something and the contact might make a noise. This is often called coil whine. If a system is designed and built in such a way that it doesn't have metal bits that can vibrate, then it shouldn't make any noise at all. I don't think I've ever heard any of my Apple computers make coil whine noises.
So I suspect the noise you're hearing is from the monitor doing whatever electronic things it needs to do to change the image (move the cursor) and not from the MacBook itself.
What can also happen is, depending on your audio setup, electronic activity could induce a signal on the input to your speakers and your speakers will make a corresponding noise. This will happen when your audio electronics, speaker cables, and/or speakers are poorly shielded. I'm sure we probably all remember when computer speakers would make noise when you received a call on your cell phone:
