I'm not sure how easy it is to replace but the ifixit.com website does have tutorials on how to fix everything with difficulty ratings so check that out. I trust them, they seem to know what they are talking about.
OK, this is weird because I just started noticing the same thing from my MBP (left fan). It's very subtile right now. I have to put my ear kinda close to hear it, but there's no doubt. Turning up the fan speeds made it obvious to me. Wow, this is retarded. My Mac's not even that old (early 2008 model)!
I just replaced the bad fan in my macbook. I took some picture of the inside of my macbook. I also upgraded my hard drive to a 320GB at the same time I installed the new fan.
Here are the pictures: http://gallery.mac.com/syclonefx#100008
I just replaced mine and ordered the fan from powerbookmedic.com. My right fan has been giving me issues since I was under warranty, actually, but it stopped and never started again until recently. (Apple wouldn't replace it because there was no noise at the time.) I got sick of hearing the noise and looked for a replacement.
BTW: Eventually the fan will stop entirely and will refuse to run!
This would be the second time I've disassembled my MacBook Pro, the first time was for the hard drive upgrade.
I recently had the same problem, and I can tell you, it will just get worse. Mine did the same thing, starting out like a quiet geiger counter, and then just yesterday it started going really loud. I only bought it 6 months ago, so I brought it to an Apple store and there fixing it right now for free.
Well my mac book pro was doing the same problem. I finally was fed up with it so I decided to see what was wrong. I opened up my mac and took out the fan. I read a bunch of things about replacing it but I wanted it to shut up NOW and didn't want to buy any parts. So i took the little fan itself apart.
The fan is kept shut by two little screws. There is a top flat lid with a motor. Then there is a case that surrounds the rest of the fan. The fan is just held on to the motor like a magnet. Just pull it off. I then looked at the different parts of the fan and saw where the fan was rubbing the case. My fan was rubbing on the bigger case that doesn't have the motor attached to it. I took some scissors, opened them up, and used one of the blades to shave the edges of the fan. The fan is just plastic to it wasn't that hard to shave it down. Because it is plastic it isn't unbreakable and I broke one fan blade off. oops. I then realized that if I pushed the fan into my carpet it would hold the blades in place as I shaved them down and they wouldn't break.
Before I put it all back together I carefully turned on my computer while it was all apart. (that was the hardest part) Sure enough it didn't rub anymore.
Now that it is all back together it is so much better. It isn't perfect. Occasionally i will hear it start to rub but it isn't very often and it is much quieter when it does rub. It usually goes away if a close a program or something small like that. I don't know what it will do in the long run but it doesn't seem to be causing the fans to work any harder than before. There are a lot of youtube clips of how to take out a fan. Anyone should be able to do this if they have a small Phillips screw driver and a T6 screw driver. I had to buy my T6 screw driver but it was only a couple of dollars.
The fan costs around $60 at ifixit. So in my opinion, it is worth paying 75 or so to have it professionally repaired and warranteed. There have been so many stories of novices trying to do it yourself that for an extra few bucks it just isn't worth it. The repair isn't easy from the pics on the web.