This might have been caused by my MacBook Air haning itself one time during resume: I manually turned it off keeping the on/off switch pressed for 5+ seconds, then rebooted.
Anyway: over time I observed that the fan was running fast without much CPU/GPU/memory/disk activity.
Resetting the SMC like the answer below, followed by resetting the PRAM and NVRAM solved my issue.
I had exactly these issues with my new Macbook Air 13. Having read this forum I downloaded istat pro and discovered that my fan was always running at over 4000 rpm and the top left part of my case was quite warm. I then followed the instructions here…resetting the SMC and after this the problem was fixed!
–jeroen
via MacBook Air Fan Noise With Lion: Apple Support Communities.