macbook fan stays on?

I have a late 2008 MacBook (alumninum). The last few days the fan seems to turn constantly--even when closed--although not full blast and the MacBooks is a bit warmer than usual. The screen is switched off when I close the MacBook. The room is not overheated. I have the MacBook pllugged into power, constanly. Hardly switch it off--perhaps I should when not in use. Any fixes for the fan you could suggest? Thanks