Well, I found what was causing that frightening sound in semi wakes.  It was the
bra on the front of the van.  Where is my dunce cap?  Odd, it really sounded
like it came from the rear...   Last night I was leaning forward when it
happened, and I could see the bra flap out at the base of the windshield.

Thanks for the ideas of checking the front grills.

Ron, Did I get a defective Bra or is this a standard thing when you put a bra on
a vanagon?

Thanks again,
Vernon