The problem is - high pitched noisy tiny sound that u can hear only using more or less quality headphones. I use yamaha rh-5ma. And it's not the problem of headphones because I have different ones (sennheiser 555) and I can test in with different players like iphone or whatever. You won't hear this sound connecting standard iphone's earphones because the problem I hear is too faint for Apple's headphones to reproduce since they are lower quality than my preferred headphones.
So to make this sound on your mbp connect your headphones and then start to play something (youtube/itunes/anything that makes sound and basically activates the sound card) and then stop it. You'll hear this high pitched noise. The moment of start/stop playback can cause some different pitched artefacts. On different machines it goes in different way. Like in my MBP I have it in right ear but it's pritty annoying to ignore it. Like a tiny moskito. In different mbp i tried it it was less amplified but in two channels (left and right).
The problem is that not everyone is able to hear this sounds because not everyone hear this high frequency. (approx 14-17khz, can't really measure it because don't have that precise recording device).
What can I do to know if my Mac has a problem that can be fixed?