high pitched buzzing noise coming from speakers

I have a late 2008 macbook pro that makes a high pitched noise. It sounds like a steady high pitched noise at baseline. when i change the volume there's additional staticy noises that sometimes sound like the noise tiny robots make in the movies. This noise comes out of the computers speakers, as well as external speakers if I have them plugged in. If I plug in headphones it comes out of the headphones. It's definitely separate from the fan noise, and it unlikely to be from the processor itself either.

Has anyone experienced this before? I'd very much appreciate any help. The noise is driving me crazy.

Additional Info: speakers work fine. I'm not sure when this issues started as I purchased this computer second hand, but everything else works fine. It seems to be related to my needing to play sound. It does seem to go away after awhile if I don't play any sound.

I know the original post is over a year old, but thought I'd check and see if there is any status update on this?

I have a similar problem on my MacBook Pro Early 2008, which appeared within the last year..perhaps shortly after I replaced the hard drive:
-The high pitched noise appears as soon as any app activates sound, regardless of whether or not it's input/output. It disappears shortly after the app is shut down or the playback is stopped.
-It occurs even when speakers and microphones are muted, while an app using sound is active.
-The sound is also audible when headphones or external speakers are connected, again only while an app is using sound.
-Input and Output functions otherwise completely normally.

I'm assuming that this is some kind of issue with perhaps an internal audio cable or the I/O system...has anyone come across a fix for this?

Just found this old thread, which seems to suggest it's the I/O board.
https://discussions.apple.com/thread/1450164?start=15&tstart=0