I've run into the same problem (buzzing, 2000rpm fan based noise on the left side of my 13" MacBookAir3,2). A few points by way of summary as I think people raising the issue are being unfairly dismissed in some of the past responses.
1. This is
not an issue with cpu load. The buzzing sound is consistently present even with ~ 90% or higher system idle. As best as I can tell it's always there as it's caused by the fan running at the standard (?) 2000rpm. This is not a case where the fan is being kicked up by running graphics/compute heavy applications.
2. At the same time, this sound was
not noticeable during the first few weeks of use. I purchased my Air in mid-December. When people say that previously their Air was "silent" I think they mean silent to all practical purposes. Before this the fan noise was simply not loud enough to draw attention. I can verify this since I regularly used my Air in exactly the same context - in a very quiet room, on my lap, running the same mix of apps (email, browser). I would definitely have noticed the sound earlier.
3. Hence, I do
not believe that this a normal, background type fan noise that MacBook Air users should simply accept. First, because I know it wasn't there in the first few weeks of equivalent usage. Second, because it really is annoying. Different people have different degrees of sensitivity to this - granted. I probably won't hear it at a large conference with the computer on my lap, or in a train, or in a busy office. However, I most definitely do and can hear it in the contexts that are important to me: in a library, in an archive, or at home. I made a conscious decision to purchase the new 13" Air because of its silent SSD and because I knew from previously owning a 13" MacBook and a first generation Air that Apple laptops can be (effectively) silent under low/little load. That the fan noise suddenly (in my case, literally from one day to the next) becomes irritatingly noticeable is really disappointing.
4. Possible causes? I'm not an engineer, so I can't claim a good answer. My personal hunch is that the fan is either not seated as tightly/correctly as it was when the laptop was purchased (although I have not dropped or knocked my unit) or else is unexpectedly wearing out in some way. It's interesting that owners of 11" units are noticing this too. Maybe it uses the same model fan? I just don't know.
5. Solutions.. Unfortunately, I can't think of any except to take the laptop to an Apple Store and make one's case for getting a replacement. I will try to do this myself here in Germany. At the same time I can totally understand why someone would want to open the case and try to fix it oneself (did this help btw?). Once you register the sound in a quiet environment, believe me, it becomes really annoying and frustrating.
It would be very helpful to hear from someone who has been able to get their unit replaced or could otherwise offer more information. It may be true that Apple staff doesn't follow these forums. At the same time, once the problem does reach their ears, I am sure that the forums are consulted to get a sense of the scale and severity of the problem. Please continue to post your concerns and experiences so we can all come to some kind of resolution.
