My fan was running 5000 rpm for hours for the first few days I had my 2012 MBA. Using iStatPro dashboard widget, I could see that my processor load was light, the CPU temps were low, and that my fan RPMs were 5000+! WTF? To fix, open Activity Monitor, select all processes and then quit bash. I've had to do this every time I've restarted the computer, but it's always fixed the issue. Sometimes bash is running at 20%, sometimes lower, but it's always bash.