I just want to add to the discussion here. I am certain at least for some of us, I don't think it's a SSD write/read or issue related to while the CPU is under load.
I own a MacBook Pro 16" (upgraded to 2.4Ghz 8 core i9, 64GB RAM, and Radeon Pro 5500M with 8GB memory, in case it's one of the upgraded components that is contributing to the issue). The sound does not sound like the first YouTube video mentioned in this thread of a SSD being written to. It has more of an intermittent sizzling/hissing sound. It comes from under the keyboard area(not the edges near the speakers) but I can't seem to pinpoint where. For me, this happens while under no load whatsoever. Just having the Mac on will make this noise. Putting it under load doesn't seem to make the noise change.
Was on Apple's chat support troubleshooting this as well. One of the things Apple support and I found was that the sound is not tied to software as it makes this noise no matter where I am, normal macOS, recovery partition, even when just at the boot selection window when just holding down option on start up.
I can't tell or notice if there's a performance hit or anything because of this, but its enough to be distracting from working on the Mac in quiet places as its a very distinct and noticeable pitch.