Not sure if this is the CPU, or something in the motherboard, or the integrated GPU, but yes, you can somehow hear activity outside the fan and the rotating disks. I clearly hear some quiet clicks when hovering messages or folders in Thunderbird both on my X1 Carbon Gen 9 and my HP Elitebook 640 G6. It's more obvious on the HP. It sounds like a silent HDD writing something, except I don't have HDDs in these computers. Possible related bug: [1]

I remember hearing activity on a Pentium II laptop (which had a Silicon Graphics graphics card) and I think on a Pentium II tower with a Voodoo 2 card too, years ago. The noise was quite obvious to me, it was more similar to the Coil Whine noise from a GPU recorded by LTT [2]

I hope I'm not cursing you if you hadn't noticed until now.

[1] https://bugzilla.mozilla.org/show_bug.cgi?id=1730423

[2] https://www.youtube.com/watch?v=HP73edpQwgc