I know enough about computers to be slightly embarrassed at having to ask this, but here goes:
Not so much with newer computers, but in the old days, you could often hear a definite rustling or rapid ticking sound not unlike the sound of cards being shuffled when there was a lot of RAM access activity going on (like the POST RAM check, also not so common anymore).
What exactly am I hearing when I hear that? What causes it?
I am guessing you were hearing the hard drive “clicking”. As the system accesses RAM it often is accessing the hard drive to transfer info to the RAM. My guess is when the system was accessing data the noise you heard was the hard drive.
Hard drives differ in how noisy they may be but they are better than they once were. Still, in a quiet room and with some drives you can still hear the drive access.
I cannot imagine how RAM would make an audible noise.
It’s the drive access you heard not the ram. Seconded.
Hence my question. 
During regular operation, I agree, the normal sound is hard drive access. But I believe that’s a different sound from what I’m talking about. I’m talking 1993-1998 era computers where often you could see the POST counting KB up through the memory as it tested it, and this would be accompanied by the very distinctive card-shuffling sound.
What would the BIOS store on the hard drive that it needed to access for a POST? Or are you saying that it was testing the RAM-hard drive path by writing between them and calling that the RAM check? I can imagine that repeatedly accessing a small patch of data temporarily cached to the hard drive for repeated read-write tests to RAM could generate such a sound.
I know what you’re talking about and I think it was the internal speaker making that noise. I only ever heard it during the POST. I think they put it in there to make it sound cool. Kinda like how all computers in movies in the 80s made noises as text gets shown on the screen.
This sound came from the speaker. It makes a “click” every xK - on a slow machine it sounds like click-click-click, on a fast machine it’s zzzzzzzzzz.
Ah, now that (even more embarrassingly) had not occurred to me. Simple audio confirmation of the activity. Reasonable.
Agree with this.
I remember that.
Was just the internal speaker tracking the count.