Back when the Altair 8800 was new, people used to make tunes by standing an FM radio on top of the computer and then coding up tight loops with different delays in them. These loops when executed cause the CPU to emit radio waves which the FM radio picked up. The different loops make it emit waves at different frequencies and so it can be made to make a tune.
Modern CPUs do this too, and I'm sure that what you're hearing is the CPU (or other component) creating radio waves that are inducing currents in some cabling somewhere - headphones, telephone cable, and causing it to whistle!
Maybe you could write us a tune?!
Jim