Quite a good article in the Nov/Dec AARP magazine titled "Listening
for Quiet".
It talks about a silent sanctuary in Washington's Olympic National
park. The article
concluded by someone complaining about the silence broken by a small
plane.

The article said "The small plane flying north more than doubles the
ambient sound,
and we react to the intruder as a threat, drawing in, tracking the
source, hunching for
cover until the last traces of engine noise finally die away."