When my front driver side CV was going out I got harmonic inside the VX and a vibration that felt like it was right under the drivers seat under acceleration. Come to fine out, since the third member is hard mounted to the frame, the vibration from the bad CV was traveling through the frame and I was perceiving it under the driver seat. I don't know why your sound goes away when you turn the heater on though. That's a bit odd.
A vibration under the back seat may come from the u-joints in the drive shaft. You can (or you can have someone) lube the three zerks fitting on the drive shaft and see if that helps. One zerk at the front u-joint, one at the slip joint in the middle of the drive shaft and one at the rear u-joint.
"The major difference between a thing that might go wrong and a thing that cannot possibly go wrong is that when a thing that cannot possibly go wrong goes wrong, it usually turns out to be impossible to get at or repair."
-Douglas Adams, Mostly Harmless