It is basically seam sealer that is used to stop the drumming, the hard part about the reinforcing in these areas is that putting spot welds in at the factory was not easy and would leave unsightly marks in the outer panels. The large areas of flat panels would have vibrated and broken spot welds so they were held together with a flexible join. If you use the wall glue that dries hard it will eventually crack and break. I can remember one of our vans had the brown wadding type stuff (very technical terminology) that is glued under the dash and plenum used in strips between the reinforcing bars and the outer panel, but for the life of me I can't remember if it was my EH van or my father's HZ, we are talking 25 to 30 years ago here!