Hope you strapped the tire down somehow. Otherwise it could be booming all over the cabin in an accident.So I got my tires up to 44 psi and put my modern spare tire flat on the trunk for a road trip... surprisingly the cabin is so much quieter with this setup. No booming or significant road noise. I wonder if all that extra weight from the spare tire suppresses the noise generated by the trunk bottom before it echos throughout the car.