Having a weird issue -

There is a buzz/resonance (not really a rattle) coming from somewhere around the back of the dash and base of windshield... Passengers seem to think that it's in the center but it seems more of the passenger side to me. Happens on certain types of concrete roads. Of which there seems to be a lot of in my area. Annoying.

I've done all the "normal" stuff - tighten down the battery, checking vents & rearview mirror etc, nothing seems to work. Anyone have any ideas of where else to check?