Put a stethoscope on the bell housing. It often will help define the sound better. It sounds like there is something loose in there. My guess, is a worn transmission input shaft bearing or the clutch fork/throwout bearing is bent/brokem. . Unfortunately you'll probably have to drop the powertrain and look in there, It's a lot of work but I don't see that you have many other options.
------------------
" THE BLACK PARALYZER" -87GT 3800SC Series III engine, custom ZZP /Frozen Boost Intercooler setup, 3.4" Pulley, Northstar TB, LS1 MAF, 3" Spintech/Hedman Exhaust, P-log Manifold, Autolite 104's, MSD wires, Custom CAI, 4T65eHD w. custom axles, Champion Radiator, S10 Brake Booster, HP Tuners VCM Suite.
"THE COLUSSUS"
87GT - ALL OUT 3.4L Turbocharged engine, Garrett Hybrid Turbo, MSD ign., modified TH125H
" ON THE LOOSE WITHOUT THE JUICE "