Inside your case it will be a little quieter too, those results are also with the stock cooler for the 5850 and 5870, OEM coolers will likely be quieter. If you play games with headphones on you likely wont hear it even with the louder stock fan on.
I hac allready seen that chart, but was looking for something to compare it too. I ended up just looking at a generic decibel level chart, which says that 40db(idle) is about the noise level of a library and 50db(load) is about the level of a quiet conversation.
However, this review claims that the 4850 has an idle of 37db and a load of 40db. I have a 4850 and it's WAY louder than a library, which has me a bit confused.
Sounds like the fan is running louder than it should. It's either set to full speed or close due to poor air circulation, hot ambient tempurature, needing to be cleaned out, or because drivers or you set the fan speed to higher than it should be.
And while it may sound near quiet, it still may be making sound, and while by it self it's quiet, adding in the gpu fan increases the sound more noticably.