tbear - 11:33 pm on Apr 25, 2005 (gmt 0)
I doubt if stopping the fan with a pencil is such a good idea......
You might press the pencil against the fan housing and put your ear to the other end. That way you should be able to hear which fan is noisy.
dmorison's idea is also good, disconnecting each fan in turn for a moment to see if the noise stops.
Since the pc is just a month old, I assume it has a guarantee. MAybe, drop it back to the dealer and let them mess with it. That way if something blows, they will have to fix it....
Good luck :)