Hi there, I've had this MBP for a few days and noticed that when I plug in headphones to the headphone jack, I can hear a fair amount of hiss and background noise. Often, this includes intermittent but clearly audible blips and computer tones. At the moment, I am using the iPod's earbuds. When I use my Sennheiser HD 515's, I can hear a single intermittent tone as well (but I don't hear as much other noise as with the iPod earbuds.) Anyone else? Is this considered normal? Sorry for all these posts... I've never had a mac laptop before so I'm not entirely sure what to expect.