tl;dr: My fan was loud. I had the power jack replaced. Now the fan is quiet. Why?
I have an Acer Aspire Laptop. For the last several months, I've dealt with a spotty power connection--the cord had to sit just right in the jack to get power. I took it to a repair shop (visit 1). The technician determined that the power jack was faulty and needed to be replaced.
While dealing with the spotty power connection, I've also noticed that the fan is a bit noisy. I cleaned it (that is one bit of laptop maintenance I am comfortable doing myself!), but it didn't help much.
When I got the laptop back from the shop, I was disappointed to see that the power connection was still unreliable, albeit better than before, and I recall that the fan was still loud. I returned to the shop (visit 2), and the technician apologized, saying that he thought the jack might have been damaged because the plug head was a bit too large, so he had replaced it with a smaller head which, judging by the resulting bad connection, was apparently too small. The solution, of course, was a simple matter of switching out the heads.
With the proper plug head back on, my computer's power supply is refreshingly uninterrupted, and, to my surprise, the fan is as quiet as the day I bought the machine! I could understand if the technician had done a little pro-bono fan fixing between visits 1 and 2, but the fan didn't quiet down until after visit 2, during which the only change that was made was to the plug head. So could my fan noise have been caused somehow by the spotty power connection?