Look under the back seat ,you will be surprised on the bounty collected over 30 plus years ,snacks are not to be included.Sometimes if someones been fooling around with splicing the orig fuel line at the tank ,the common use of 2 fuel line clamps are used ,when enough road vibration gets going this could be that sound ,that is if one of the clamps has lossened up over time. The sounds that emmit from the rear end area are usually clunks and hummms ,you have what is a new sound which deserves a new name,the kuene sound.