While were talking about noises from the back of the car check to see if the trunk lid torsion bars have a rubber sleeve between them where they crisscross in the middle. If the rubber is missing the bars can rattle against each other.

I don't think the trunk torsion bars could create the exhaust drone, but you may as well add the rubber if you don't have it. A couple inches of fuel line with a slit down the side of it will duplicate the original just fine.