Here is today's reply from Apple..
Apple engineers claim they are unaware of the buzzing/hissing sounds emitted from Airport Extreme Base stations (either from the 4th generation or from our new 5th generation 1408 model). In short, they dismiss all the similar complaints posted on this forum in 2009, 2010 through May 2011.
However, the engineers would like to study my AEBS. In return I will be sent a replacement device. The wrinkle here, according to the Apple rep, is that this replacement will be a "refurbished" 5th gen AEBS. And since this 5th gen model went on sale only about 2 weeks ago, it is almost certain that this so-called refurbished device will not have addressed the buzzing/hissing issue.
Thus, the Apple rep suggested it might be best for me to retain my current AEBS. He kindly offered to call me in 2 weeks time. He thought that in the interim they may receive additional, similar complaints from other users.
Here's hoping.