I found this on another site, see if it sounds familiar. They describe it as a howl. see link below for full thread.
You may have to join to view, but it is free and painless. Decent site.
http://www.babblers.org/forums/showthread.php?t=9838
In my case it turned out to be a bad right rear wheel bearing. If you stop by the dealer or other mechanic they should be able to diagnose it for no charge. I had hoped it was just tire noise as it didn't seem to change much based on side loading. A test drive at the local dealer and subsequent inspection of the tires showed it most likely to be the wheel bearing and $500 later the noise was gone.
Here is another "hissing" sound thread:
http://www.babblers.org/forums/showthread.php?t=9998
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