http://www.karlmacklin.com/diverse/mousewhine.mp3

A few seconds of ambience, then a few seconds where I hold my mouse to the microphone (audio-technica AT3035).

Yes, my mouse, a logitech mx510 has built in coil buzz/whining.

I was stretching out after a few hours in front of the computer, and found myself in a position where I heard this noise. Naturally, I stopped mid-stretch to focus in so I could figure out where it came from.
At first I thought it was my monitor, since it makes a noise too (but not of that character).

Guess my surprise when I zoned in to the mouse as the source

Sounds bad... but how loud is it?

Not that loud, I can only hear it within 25 cm, if I aim my left ear straight at it (my right ear hears high freqs slightly lower).

Since the mouse is always on the right side of my keyboard, it's not a problem.

I actually put it on the left side to try it out, and it was actually quite annoying

Nothing new here. My Logitech mx500 does the same, and many others have reported this problem on spcr as well, a couple years ago.

I didn't notice it until a few weeks ago, but my trusty MS Wireless Intellimouse Explorer 2.0 also makes a high pitch sound while moving the mouse around, though it seems intermittent (session-by-session).

I also remember the thread specific to Logitech mice and someone who complained to their tech support. Did anything ever become of that and do their new products still have the issue?
(I like their wireless "Wave" kbd/mouse)

Had one of those squealy MX510, bought a Razer Deathadder to replace it, haven't had a need to look back.