I wanted to amend this post after looking under my desk and seeing a lot of dust. Basically, you can be assured regardless of your fan noise, if you have dust under your desk, your computer needs to be cleaned.
My PC has been quiet and performing well. But this is what I found after cracking it open and removing the fan (4 screws):
Just hovering the vacuum over the fan immediately resulted in this:
Also, don't forget to vacuum out your power supply from the inside, and outside:
You may have to take your video card out as it gets dust as well. Be aware, there is a clip at the forward base of the slot that holds it into place. You will need to push this little clip in to pull the card out. Also, if you remove power cables from your video card, don't forget to put them back in when done.
Again, remember, unplug the power, keep one hand on your power supply while working, don't touch any circuits with anything, also it's a good idea to work on a hard floor, and not a carpet (static electricity).