I have a 73 Nova hatchback and get ALOT of noise from the hatch area when driving. Mostly rattling and squeaking from the sub-floor. What have you all done to the sub-floor/hatch area to at least make it a little more quiet?
Get some 3M rope calk and where there is metal to metal contact on panels, apply a bead of the calk and then resecure the panel back down. If the screws do not pull the panel tight then install some slightly larger screws to hold them in place.
The 3M calk is something like dum/dum or playdough and can be formed using your hands. Unlike other things out there this stuff is designed to stay flexible for longer periods of time and not dry out.
The calk comes in a little box and should be available at the better automotive paint supply locations. It comes in I think three colors but black is the most common.
Another source of noise could be from the false floor support rod. Make sure the retainer clip for this is tight and when the rod is clipped into it's storage position it doesn't rattle on the pivot end.
I've also seen stuff get in between the rear side panels and the clips on the outside edge false floor extensions.
I think too an option was a carpeted rear hatch floor area. Having this will also help muffle any noises from below it.
I had a 74 Nova sedan once and from the first day I got it, it had this annoying squeeking coming from the boot area. I tried everything to try and find where it was coming from !! Even got in the trunk and got the misses to drive around the block so I could find the noise in the back but couldn't find it. It was 5 years later that by chance I was doing something on the back parcel tray and noticed the rear windscreen moved then I leaned on it. Had a closer look and the sealant holding the window in place had come unstuck from the glass and the glass was scatching against the chrome trim on the outside and that was the only thing holding the glass in place!! Man was I happy to find that.