High Pitch Whistling Noise?

Does it happen all the time, or is it only when the computer is under heaver loads, ie, moving large files, burning disks, rendering, ect. Does it occur as soon as the computer is booted, or does it take some time, As if it takes some time, it may be a fan that is faulty, or a fan that is simply loud. DVD's and CD's can produce a whistle if the drive spins up and down quickly resulting from an unreadable disk or a disk it does not know how to handle. Any information pertaining to the situations when this happens would be very helpful, Thanks.
-Trev

Does it always happen when fewer apps are open, as if it is only when you have multiple apps open, it could be that. Also, where about from the laptop is the sound coming from, as localizing it will help to solve the problem. What is your current CPU and Memory usage? (found in the Activity Monitor). Anything else about when it started, circumstances regarding its start, or any other ideas would be helpful, even if incorrect. Thanks
-Trev

If it whistles a different pitch or intensity by lowering or raising the screen brightness the culprit could be a loose transformer on the backlight's inverter board.

Have you tried logging into another User Account. If doing this cures the noise then it tends to eliminate hardware as the problem.

In Energy Saver Preferences set the hard drive to go to sleep when possible. Then quit ALL apps and sit and wait till the drive spins down. You should be easily able to hear that if you can hear your hard drive. It sounds like a quiet fan somewhat. If the whistle quits when the drive spins down and then returns when you spin the drive back up it strongly suggests the drive as the culprit.

Is the noise coming from your speakers. Mute the speaker audio via the F key and see if it stops.

Try describing the whistle as accurately as you can. Describe the picch, regularity, e.i. is it smooth or rough etc.. Does it come on then off randomly or does it stay on once it first comes on.

There are a lot of things that can make a sound that can be described as a whistle but they can all be quite different from one another. Saying simply a 'whistle' is to vague.