1. Are the bolts tight? (sorry. had to ask that one)
2. What's the condition of your weather strip?
3. Is it in roof panel original?
4. Is this a recent occurence?
5. Has the car ever been in an accident or had bad stress applied to the body?
It may actually be the roof rattling but also check the rear hatch. If two bolts holding the glass to the hinge becomes loose its easy to mistake the sound to be comung from the rear portion of the roof.
There's a bolt plate that attaches to the windshield header with two screws; check to see that this is also tight.
When tightening the roof panel, I suggest snugging up the two rear screws first, to the Targa bar. Then tighten the two to the windshield header. That pulls the roof panel rearward up against the targa bar, and then down against the windshield header.
My '89 has never rattled except when I first drove it home. I tightened everything up then, and the rattles all went away.
Standard disclaimers, YMMV, etc.
I had a problem with the brackets on the top getting loose... basically you would tighten the 4 main bolts on the top, but the brackets holding them to the top weren't tight, so it was actually loose, causing a rattle.... Try tightening all four brackets on the top while it is off... there are two bolts per bracket, and your top wrench should work to tighten those bolts as well (it may not work on the rear bolts though)
I've seen these rubber strips that were OEM on '92's?-to '96 that are sold by several of the larger aftermarket companies to retro-fit the older C4's or as a replacement in the newer ones. ($10-15 I think)
Supposed to keep the rattlin' to a minimum!
This may help if tightening the bolts doesn't completely eliminate that annoying racket!