I have been used not to hear when the MP 6.1 was working...or I believe it was so. Sometimes just the light showed me from a certain distance that I had forgotten to turn it off. After 2 years I can now hear distinctly when the computer is on and when it is off. I don't mean put it under any stress but simply by turning it on I hear it is on. I suppose that what I hear when it is on is the fan since it is the only moving part. The MP is on my table roughly about 50cm of my ears. I would be glad to learn if other people hear it as well when it is on, to know if this is normal. It is not any high frequency noise, just sounds like a low frequency sound made by some appliance turned on,...what actually is the case. I am not truly sure...but believe that that sound was not so audible before. The Apple hardware test gave no problem, but I don't believe that the sound level is part of that test. Thank you in advance for any feed back.