I've spent a great deal of time trying to make my 02 wagon sound less like the cheap-interiored car that it is. To that end I've raamat'd the whole thing and done ungodly amounts of work getting rid of every rattle I could. I went so far as to put together a testing set consisting of a tranducer, power amp and signal generator so that I could sweep over large ranges of vibrations to find parts resonating.
But I've never been able to silence a low-frequency vibration/rattle from the hatchback. Last night I was closing the car and tripped, falling against the hatch. I noticed that as I removed my weight I heard a slight, but annoyingly familiar clunk. Turns out there was a slight bit of slop in the latch/loop connection. Only when you press hard on the hatch (or hit a sharp bump) would the connection get a little slack and clunk when springing back. I loosened the bolts on the metal loop and slid it as far forward as possible. Then I loosened the latch on the hatch and slid it back as far as possible. My car is now silent over bumps. It took me four years and 70K miles to figure this out.
Just thought I'd share.