Sony VAIO model pcg-R505TS fan

I have a sony VAIO in a docking station and the fan speeds up and slows down intermittently as I use the unit. It sounds like a car going uphill and then downhill sometimes. Is this normal? Anyone has suggestions on this? I tried various power saversettings. The unit is plugged into the wall outlet. It has 128 megs of ram running Windows ME and has lots of drive space.