knocking sound in car...

i drive a 1991 gt...i have a certain slight knocking sound coming from what i think is the transmission mount...its not the harmonic balancer cause motor is smooth when revved...the steering column hits on my long tube headers but there is still a slight knocking sound at idle and at anywhere from 2000-2800 rpms...any ideas? transmission mount? driveshaft? let me if you have any ideas...thanks!

sorry my computer is screwed so i have two similar posts to this one...there is an intermittent knock...if the wheels are pointed straight, you can feel the steering column hitting the header but when you turn it there is still a slight knock in the chassis and a slight intermittent knock when i slow down from highway speeds...i never checked for an exhaust leak...i may have to check that out