I have a cracking type noise coming from my iMac occasionally. It is not constant at all and when it occurs it only lasts for a split second. I hear it about once every 15-30 minutes and it has been occurring less frequently tonight as I have the machine running longer. Is it just some sort of settling of the components because the machine is new? It seems to be coming from the rear of the machine, thought it is hard to tell. When I hear the sound the machine does not act differently in any way. The computer works flawlessly and is silent otherwise but I am wondering if it is anything to be concerned about. Thanks.