I have to say I've never heard that sort of noise coming from any kind of computer, but it definitely doesn't sound good So, you may want to seriously consider taking it to an Apple Store (if possible?) where they should at least be able to diagnose it free of charge. Then after that they will first let you if it is even fixable, and if so, they should also provide you with an exact estimate of how much it would actually cost for them to repair the problem. However, I did google 'mac laptop making buzzing noise' and found that you're certainly not alone, so you might wanna start there Good Luck!
yeah well one thing is certain, it is not suppose to make that noise.
You say you can adjust how loud it is so its obvioulsy coming from the speakers (that would make sense atleast ). it could then either be strong electromagnetic interference, like for example when you hear speakers crackling when your phone nearby is recieving a text etc.
Or it could be a faulty speaker.
I recently drove under some powerlines and my MBP aswell as iPad and iPhone starting creating weird noises from their speakers when they were in my bag.
I misread the problem...thought it said you couldn't adjust the volume of the noise!
Does it do that on battery power and in any location? If so, there's some sort of electrical problem inside the MBP that's announcing itself that way...like one of the components has gone a bit haywire. Could be something in the audio chain itself.