1998 Windstar, buzzing from driver's side rear @35-40MPH

I've trying searching this newsgroup with Google for some hint as to what might be wrong with my niece's Windstar and I've searched the
nhtsa site for applicable technical service bulletins, but I've come up dry.
Between 35 and 40MPH, a buzzing sound appears and it sounds like it is coming from the driver's side rear. The noise only appears in this speed range.
My first thought was that it might have something to do with the gas tank vapor control system, since the gas tank filler tube is back there and the buzzing sounded like it could have been a leaking air valve. So I loosened the gas cap and drove it around, but the character of the sound did not change.
It's a front wheel drive, so there's no differential. I would not think that a problem axle bearing would make noise at only such a specific speed range.
Does anyone have any other ideas?
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