After installing Mavericks on my mid-2011 iMac, I've been getting occasional sounds from what I believe must be the hard drive. Every half hour or so there will be a 'tick' and then subtle sound of powering up, before fading out again. All happens very quickly. It used to make this sound way back when I first got the Mac, but the noise disappeared completely in Mountain Lion. Now it's back in Mavericks. Has anyone else noticed this issue on their machine?