I had a weird experience today.  I am trying to sell my 2002 Focus Wagon,
and a guy took it out for a test run on the freeway.  He came back and said
that the car ran great, but it had some kind of tremendous vibrating noise
when running at speeds higher than 50 MPH.  I had never heard it, so he took
me out to show me.

The noise is mainly there when one of the rear passenger windows is open.
No wonder I had never experienced it, as I never have any windows open when
driving the car.

I am not sure what it is, but the noise (kinda like "chug, chug, chug") is
quite severe and increases in intensity, but not frequency, as the speed is
increased past 50 MPH.

I think it may be  some acoustic thing, caused by air buffeting in the cargo
area (the tonneau cover there was retracted, and I have not tested it with
it extended).  It gets to be very loud, though, and we could feel the
pressure waves in our ears.

Have any of you here experienced anything like it?  Any ideas?