Asus A4G going noisy

My problem is: I have now 3 years (maybe more) good A4G Asus, its performance is excellent but now makes a lot noise that is unbearable.

How to describe it ...
The fan is almost continuous but it appears to be heavy (at least until the computer goes quiet) than I have noticed that the buzz begins about 30-40 seconds after loading windows and never stop. Then the situation becomes unbearable when I start to urge some of the system application, eg autocad and maybe winamp, the fan starts to run at maximum, the notebook is warm (very) and there is a unbearable noise.

To avoid the heating I "raised" a few feet behind the cabinet so that more fresh air to enter but it is certainly not the best solution.

How's the way you've blown all the dust from cracks in a compressor? Especially the crack from where it should leave the air blown from the CPU fan.

Well if you want to do a job for good, open it from below, clean well the bags to the CPU heat sink, proc and the heatsink thermal paste.

Strange concept of noising, my laptop Asus has 3 years (Turion64, 2 GB of RAM)
Anyway I had a similar problem after a while the laptop became hot and I think that the temperature has contributed to the premature death of the disk of the laptop. I also ordered one of the bases of Antec with the fans raised to keep the laptop cool, but I returned because I had sent the wrong model. At the end I opened the laptop and I "cleaned" with a compressor (did not do as I have done that I fired the jet to 3 cm from the fan and the laptop has moved on the desktop through the power of the jet)

Meanwhile, thanks for responses, try to do as you say.
But it may not be the fan? In fact if I approach the blower from which leaves the air and you hear distinctly the fan running without making too much noise.
The buzz seems to come from another party, such as the center under the keyboard. There are other fans (type of video card ATI9700) or something else that can make noise continuously?
The computer is always noisy, even when turned on, even if it is not doing anything and even for a few minutes after being put in standby!