I was thinking maybe the little "bump stops" are adjusted in correctly and it's going too far into gear?
1986 GT with the typical poor boy mods. 3.73s, Hurst short throw, King Cobra Clutch with aluminum quadrant, lowered two inches on Eibach springs, Hopefully adding some heads and a decent intake soon!
my 95 v6 used to do that if I was stopped at a light in 1st, and usually in 3rd or 5th depending on the rpm. usually right around 1-1500ish. I always figured it was just harmonic vibrations.. does it do it depending on rpm or weather your stopped in 1st? does it change with the clutch in?