Fan noise is usually caused by dust buildup on the fan blades or bad airflow from dust stuck on intake holes. I have a PC with a variable speed Thermaltake fan on the CPU, and over time it gets dust built up on the blades and starts to make a loud humming noise. All I do is brush the dust off and no more noise.

I would pull it apart if you can and clean all the dust out... my eMac had a HUGE clump of dust stuck on the CPU heatsink behind the fan. I cleaned it, put Arctic Silver on the CPU and reassembled it. Runs cooler than it did when I got it.

Same thing with my MDD... the front intake under the port holes gets dust buildup faster than anything I've ever seen.