Spare tire thud

[ChevyHighPerformance]

When I'm driving over a washboard road at a certain low speed, I can hear a low frequency thumping/thud sound from the back. When I pound on the spare tire I think I hear the same sound. Everything is tight (wheel and spare tire mount), but when I hold the top of the back door (I can feel it vibrating when I hit the tire) and hit the tire the thumping sound is reduced. Anyone else notice this?