I’ve noticed this occasionally. When I’m typing in Textedit (my text editor of choice: so fast, so easy) there’s sometimes this quiet, high-pitched ‘squeek’ that comes out of the laptop even when the volume’s muted.
I noticed that it only happened when I was typing at least two lines above another, which didn’t explain a thing!
Then I came across this thread which gives both a reasonable explanation and a solution to the problem. I’m now running on “Automatic” Processor Performance and am squeek free.