My new iMac 27" (i3, 3.2GHz) is not so quiet as my previous iMacs - I can hear a low humming noise! Not very loud, but I don't like it. It grabs my attention... It sounds like a fan motor or a transformer. I can also feel a corresponding vibration. Is this normal now? Does anyone else have this problem too? Would you recommend to return/exchange this iMac? Thanks!