This happens to my Macbook almost all the time. My computer will be idle and the fan goes up to the max. I check the activity monitor and it shows that 1 program is using a lot of CPU...programs like Firefox, MS Word, or iTunes. This would happen even if Firefox was just sitting idle on any webpage..it could be google.com and it would do this. On MS word, just a document would be open. On iTunes, just the library is open, but no song playing. I don't remember my Macbook always being like this, but don't exactly remember when this started.