When I first wake my iMac up from sleep or turn it on, it makes a knocking noise that kind of sounds like a light clap. It only does it once, and only does it when the computer hasn't been used for a while. I first noticed it a week ago when I woke it up from sleep, and it now does it every time I turn it on or wake it up. It sounds like it is coming from the hard drive. There are no indications of the hard drive being bad, except it has always been noisy. It is a mid 2010 21.5". Should I let it go, or take it into the Apple Store? I only have a month left on my warranty, though I plan to extend it to 3 years.