wierd noise from...?, frustrated/pwnt by bumps Options

[Corvius]

'94 gt coupe. Everytime I go over rough road surfaces I hear a very annoying, somewhat dull "thud thud," coming from *somewhere* beyond my back seats. It sounds like there is something loose (like the spare tire) that is moved only slightly when I go over bumps...but I can't seem to find where it's coming from. Hypothetically if I was driving on glass and wipped the car back and forth, I would *not* hear the noise, so I don't think it's anything loose.... I tried taking all that plastic/carpet liner out from the trunk, still a noise. I tried putting carpets under my spare tire, and made sure there wasnt anything else in the trunk that was banging around (jack is wrapped ina big towel and secured with rope), still a noise. My last venture was to cram towels in the space just under the highmounted brake light, but I still got a noise. It doesn't sound underbody, it sounds inside the cabin, but what do I know...

Anyone else have/had this problem? Any suggestions?