I bought a 2.66 GHz Quad (2009 mac pro), GT120 and I today noticed a continuous sort of 'purring' sound for 2 seconds, and then 2 seconds no sound. This sound occurs even after sleep and I first suspected one of the hard drives, however there is no writing/reading activity when the sound occurs. a. Is this normal (i.e. do other users have it)? b. Any suspect?