A friend was over and was using his mod-2010 unibody 15" MacBook Pro. I noticed it was making a loud noise and asked how long it had been doing that. He answered "A while but I got used to it." After yelling at him and making him back it up, I took it to my bench and opened it up. With a can of compressed air, I immediately confirmed that it was the left fan (probably the bearings) and could show him using the air alone. Time for a new fan!
Except for the one stripped screw on the case bottom, the repair couldn't have gone easier. I had looked up the iFixit repair guide before ordering the correct fan. The guide specified which Torx bit I would need, so I dug that out, and also recommended a Spudger (which I've always wanted) so I ordered that along with the new fan. I will admit I shopped around for price on the part, and considering shipping cost and speed also I found that iFixit was the best deal. The parts arrived much faster than predicted, and thanks to the spudger it took about five minutes to replace the fan. Having the iFixit Magnetic Project Mat to hold the screws made it even quicker! End result: The MacBook runs much quieter and cooler, and the battery lasts longer.
I'm an experienced computer troubleshooter and repairer, but I think pretty much anyone with even a tiny amount of manual dexterity could have diagnosed this issue with the help of the Internet, and using the iFixit guide have easily found and ordered the correct part, and installed it quickly and easily without help. The iFixit guide lists this repair as moderate difficulty (probably because of the need for the Torx screwdriver) but it is really easy if you have the proper tools and can keep track of small screws!