Wow, can't believe I missed this one, making PCs quiet is my version of crack cocaine!
Right, as was said fans get louder over time as they get worn and dust gets into the bearings (sleeve or ball). It most certainly can NOT hurt to replace the fan, I had a fan fail once and was lucky that the processor survived (was back in the days of the slot-1 Pentium-2, not too much heat)
You will find that there are quieter fans out there than your current one. Panaflo and Papst are often heralded as the "reference" fans, but there are many other good brands. I can't recommend Zalmann, I had one of their fans and although quiet at the start it was rather loud one year on.
If you replace your noisy fans and are still miffed at the noise, you might want to look at a "fan controller". This is a small unit that controls the speed of the fans via a rotary knob and is often incorrectly referred to as a "rheobus". By slowing the fans you do of course reduce their cooling effect, but this is not a problem provided you don't slow them too much.
Hard-disks can also be surprisingly noisy.
I am in the process of improving my super-silent PC at the moment (Athlon 2200 with only one fan in the whole system!!!
). Wait until I get paid this month, then I will show you what a REAL heatsink is!