Transmission tunnel, trunk pan, rear wheel wells. Would also guess a spritz anywhere else that would reasonably be expected to 'drum' at road speeds.

There were also asphalt mats inside the cab that baked into place with the sealers. Definitely one under the rear seat of our '68 LTD -- which my Dad found with bare hands while trying to fish out the seat belts that had gone MIA. And just to compound the pain, it was with his Mom as a passenger. Oh, and this discovery was in July, in Gettysburg PA, on the main drag, when it was about 90F temp and humid as a locker room. The entire rear seat cushion had to come out and somehow, we fashioned a retainer to keep them from slipping back under. I

I don't think poor ol Pop came back to earth orbit for a few hours.