M35 - something rolling around in the gas tank??

[Mobert]

09 M35 with a little over 37k miles. Recently started hearing a rattle when driving over rough pavement. While investigating I noticed that when I rock the car I hear a sustained rocking/rattle for 15 to 20 seconds between the rear tires in the vicinity of the gas tank. Given the duration and tempo of the noise, it sounds as if there`s someting rolling around in my gas tank. Has anyone else experienced this and if so, what was the cause?

Thanks,
Mobert