I have a 2011 MBA and when I normally use it the fan never comes on. I can run Safari, iTunes, Xcode, etc and it doesn't get hot. In the past few days the fan always comes on pretty soon after I start using the laptop. This is with light usage - it runs about 4500rpm and gets quite hot & uncomfortable with just Safari and iTunes open. Is this a hardware problem? I've tried resetting the SMC and using a vacuum near the vent. Maybe it needs to be opened up?