Did you check the oil
Old cars - Old tactics. Get a long screw driver Put it against every area/part of the motor and put the other end against your ear. It will be noisy but it should tell you if it's at the front of the motor or back. If you can get the car on a hoist you should notice if it is louder underneath. Try the screw driver every where including the timing chain cover, oil pan and trans pan. Might as well try the axle and posi bearings and trans tail shaft while you are there. Disconnect the speedo. If you get desperate pull the trans. It's not that hard when the car is up in the air. If it turns out to be the throw out bearing or bronze bushing you will be pulling the it anyway. If you still can't find it do it all again with a piece of heater hose. It makes noises more subtle then the screw driver.
Good luck.
PS. Check for a smart ass mouse in the glove box