MBP Fan Noise Problems

A year ago I replaced my left fan because it was making loud rattling noises that increased to very loud at higher RPM (playing WoW, running video-editing software, etc). Now, at the time, I was fine with it. I do use my computer a lot, and replacing a part was to be expected. However, recently the left fan has started making the same noise. Wouldn't it make more sense for the right one to go next? Or could it be the way I play World of Warcraft causes the left fan to have problems (I keep my left hand on the WASD keys - could this cause some harmful weight on the left fan?) Also, I usually use my computer with it on my lap, but I do have a lap pad/stand. Would it not being on a 100% level surface cause problems?

Thanks for any help you can give!

The left fan is nearest the warmest running components on the system, so it may spin more consistently at a higher RPM.

With that said, fans are mechanical in nature and it sounds like the bearings are failing (which is the noise you're hearing). So, there can often be no rhyme or reason for a failure. It's roughly akin to taking two cars of the exact make, model and year. You can run them side by side for 10 years in the exact same way and come up with different failures, when logic might suggest they have the same issues.

I'd replace it with a factory new part. I'd also recommend getting in there at least once a year or so and cleaning the blades with some compressed air. Make sure you hold the fan still while you do that (you don't want to overspin the fans).

Thanks for the quick reply! I think I will try a good cleaning to see if that helps before I get the fan replaced again. It's not nearly as loud as the last time I replaced it, so hopefully it's just dust build-up.