Aluminium Macbook noise

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For about a week now my Macbook has been somewhat noisy. The first time it happened I was using it in my lap, so I wrote it off as it being the fans acting up. I let it cool down for the night and picked it up the next morning but there was the same noise (a sort of 'brr', back and forth, lawn-mower-like noise).

I searched for possible solutions and have tried the following:

-Checking the printer queue for any documents, since this apparently causes the fans to spin more. There were no documents.
-Checking activity monitor to see if there's any app with excessive CPU or memory usage. Firefox does take quite a bit of both, but not above 20%.
-Resetting the System Manager Controller. It works for about one hour, and then the noise starts back up.
-Opening the laptop and cleaning the fan. It was fairly clean, but after further cleaning the noise did not stop.
-Running Apple Hardware Test. It did not find any errors.

I also installed iStat Pro to keep track of the system temperature and how fast the fan spins. I've found the fan stays between 1800 and 2200 rpm and the CPU temperature oscillates between 50C and 70C. I don't know if this is excessive, but it doesn't seem so to me. The noise will happen at all temperatures and fan speeds I have been able to observe.

Sometimes closing the lid and then opening it back up seems to help, but only temporarily, and then the noise will start as a sort of clicking and progressively get faster and noisier. Restarting will do the same, but sometimes the noise starts even before the login screen loads.

I'm thinking it might not be the fans after all, but the hard drive, but really, I have no clue what it is or how to fix it.

Any ideas?

My computer is a Late 2008/Early 2009 aluminium Macbook with Snow Leopard installed and a 4GB memory upgrade.
 
Make sure that no CD is in the CD drive.
If you watch iStat Pro, then run some software that will cause the processor to heat, and you will hear exactly what the fan sounds like. and you can verify that noise is from the fan.
A good test for the fan is Google Earth, where you can run the demo tour. That will really cause your system to heat up, and the fan will eventually go to maximum.
The CD drive, the hard drive, or the fan are the only real noise-makers in your Mac.
 
If you watch iStat Pro, then run some software that will cause the processor to heat, and you will hear exactly what the fan sounds like. and you can verify that noise is from the fan.
A good test for the fan is Google Earth, where you can run the demo tour. That will really cause your system to heat up, and the fan will eventually go to maximum.

What I always do to heat up machines is run 6 games of chess with the preferences set to stronger and play computer vs. computer. Then to add to that I'll run 3D grapher with every example launched. It will stress out any machine and max out the fans. Otherwise there is some 3rd party apps out there that will let you manually control fan speeds.
 
Hmm. No CD in the CD drive.

I tried the Google Earth thing plus a game of chess and the fans did go up to about 6000rpm, and I hear a smooth strong hum but over that I also hear the vibrating noise, and it does get stronger. But adfter the fans go back down (according to iStat Pro) the annoying noise remains. If it IS the fan, is there anything I can do short of replacing it?
 
If you are sure there is nothing in the optical drive. If it is the fans they are pretty easy to replace on those units. If the machine is still covered under Applecare, take it in. If not the fans are relatively inexpensive, but on the downside you are not going to find one very easily as they are very proprietary. So you may need to bite the bullet and let an Apple authorized shop fix it for you.
 
Absolutely sure. Awh. Replacing/getting it checked is sure going to be a problem, since there are no Apple Stores where I live.

Thanks for your input!
 
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