I have had the problem for a while and it bothered me like crazy until I found that it was caused by the plastic piece where the defrost vents are - where the dash meets the windshield (I'm guessing this is only applicable to pre 99s).
To see if this is the problem, apply pressure to this piece while you drive. You should be able to immediately stop it by simply touching it lightly.
Initially I thought it was the entire panel where the airbag is located because if I pressed it down hard enough it would stop. Well it's not.
You can simply shim the defrost panel between the windshield permanently with cardboard, kleenex, or whatever.
