Are you sure it's coming from the rear shelf? My exhaust hangers were going bad (in fact, one just disappeared), and they created a sound that appeared to be coming from under/the rear shelf. These things are made of plastic, so they degrade. I replaced one and it helped a lot. Wish I'd ordered two the first time around, cause the other one has gone south...
best, whit
EDIT: there are four exhaust hangers, two are metal (trans and muffler), two are corded rubber (like a tire) and support the mid sections.