Laptop cooling fan noise is indeed annoying. One of my laptops, two years old Fujitsu Siemens Amilo with AMD Sempron had already quite loud fan noise to begin with and recently it started crying out so loud that I couldn't bear it any more. One of causes was of course accumulated dusts around and over fan itself. So I got Spray Duster (canned air spray in high pressure) as well as Q-tips with dust cleanser to thoroughly clean them up. Though those stuff may cost more than buying new replacement fan itself, because I spent almost 15 bucks altogether for those cleaning aids, and new fan could be as cheap as couple of bucks if you happen to find one. In order to clean them, you must detach the fan from housing, because dusts may well accumulated at the bottom of fan, each blade of fan as well as CPU itself and surrounding heat sink inside curvature of housing where fan being mounted.
Anyway, after clean them up, remounted the fan and turned the power on, it was of course much quieter than before clean them up, however there was still unwanted vibration noise in exist which is relatively noticeable and it will still affect audio recordings etc as background noise when I record in the same room.
So, out of curiosity, I took the fan off from housing and let it spin by itself, and then there was no noise at all (if it was noisy, then the fan is bad), fan span in complete silence like a charm during boot up sequence which is quite heavy CPU load so that the fan must be spinning at the highest mode! Now, it is obvious that there must be something wrong with this housing chassis, what happens is that if the fan is spinning in high speed and if any part of blades touches even slightly at any part of housing walls or CPU, that sound would be amplified as audible noise. So, first, I cleaned tips of every blade carefully that there's nothing remained over the original length of them as well as cleaned up one more time inside of housing where may be close to the edge of spinning blades, both sides and bottom area meet spinning blades in close distance.
After that, I heard very slight improvement, but not satisfactory. So, the last thing I could think of was that the screw holes on housing chassis must be out of alignment to mount the fan at the exact center of housing. My fan had three screw holes to mount them securely, so I only used single screw so that I could change the angle of fan to discover the quietest spot (one screw at a time), after tweaking and moving angle of fan for different directions, finally I found a sweet spot using only one of screws that there was no noise or vibration at all! :)
Now, it is even much quieter than when it was brand new! At this position, the remaining two screw holes of housing chassis are at least ½ millimeters off from the holes of fan, so it revealed that whoever was manufacturing these series of laptops had a false alignment for drilling process, probably in automated process and drilled thousands or tens of thousands of faulty housing chassis and their quality control department decided to just let it go for probably economical reason, so here they are, infamous noisy Fujitsu laptops were born and launched to the market couple of years ago!!
As for the final addition, in case your fan makes noise or performs erratically (i.e., fan stops, or makes noise all of a sudden while spinning), it may need little help of lubricating and basically you can apply any vegetable cooking oil for this. First peel off the covering paper label placed over the bearing enclosure at the back of fan, even though some material may still covering up bearing enclosure, you should be able to see the tip of motor shaft and you can drain the oil from there into bearings. Either use q-tip or tissue paper (you must twist one of corners to make it like a string, if you're using tissue paper) and then soak the tip of it with about one half tea spoonful (or even less) of vegetable oil, then place the tip of it over the tip of shaft of motor downward so that the oil will drip in to motor shaft while you're spinning the blades of fan by your finger to make sure it would be drained in to bearings evenly. After doing so for 10-20 seconds, wipe off the remaining oil dripped/spilled over the outside of fan, placing the covering label back. That's all, and you should see the improvement right away! My CPU average core temperature dropped at least 10C applying lubricate to motor shaft of 2 years old cooling fan.
Here's a neat and extremely light utility to monitor your CPU core temperature and I'd recommend you to have it:
http://www.alcpu.com/CoreTemp/Every CPU core temperature varies and this monitor may not be entirely accurate depends on your system; it is still useful to compare how your CPU temperature is, time to time.
Here's some list of core temperature for different CPU for the guide.
http://www.pantherproducts.co.uk/Articles/CPU/CPU%20Temperatures.shtmlCPUs are quite strong and stand against quite long time of abuse by over heating before they get fried. However, if you see average core temperature is constantly exceeding 90-100C for hours or days, and some part of your laptop is very hot by touching it, that's a bad sign and cooling fan is not working properly requires above fix or replacement before get to the point of no return! If it averages below 70C, it should be normal and not to worry on any CPU.