When I shut down my iMac Core Duo, I'm hearing this sound that sounds like the hard drive is spinning down. I've never heard that before, is this normal? Its also taking longer to shut down completely. After the hard drive spins down, it takes about 15 seconds and then shuts off. Does anyone else experience this? It seems to be new to me, though I realy can't remember. I'm looking for peace of mind here .