Fan/White Noise From Mac Book Air

Hi,

My MacBook air is noisy. I sat down closed Safari and opened Pages. Started typing and there was no noise. About 40 minutes into use there's a loud fan noise/white noise coming from my computer. Maybe it's protective and keeping the computer from overheating, but it's really loud. The white noise was there for about an hour and then gradually went away. This happens intermittently with use of my Mac. Anyone else experience this? Is this normal?

Thanks,

Dryersheets