I got my 2.0ghz aluminum macbook a little over a month now and I just recently notice a humming noise from the fan (rpm ~2000). This is my first macbook and I wonder if this is normal? (should I bring it in to Apple store and let them have a look?)
I bought my MB alum a day or so after the keynote announcing them and my fan still doesn't make any type of hum when idle. The fan is fairly easy for Apple to replace if yours starts to bother you.
Set your fan to about 2000rpm and sit in a quiet room.
You'll hear it.
I hear a very faint hum. But the room has to be dead silent for me to be able to hear it.