Well, I guess I may have had the hissing sound mystery solved, but only after having my exhaust leak repaired. A few weeks back, when I first posted this thread, I had taken my car over to Bob Amos to show him and see if he might have known what was causing the strange sound. Bob did notice that there was a major vacumn/exhaust leak coming from in between the intake and exhaust manifold right below the carburator. Took it to my mechanic and told him about the leak and he also noticed the big exhaust leak coming from where the heat riser was. Long story short, had the leak fixed and the the whole plugged up where the heat riser was and now I seem to have better performance, not to mention less bogging down in thrid gear. The hissing sound hasn't shown up, yet. So hopefully that took care of it. Still have a little rough idling, but I guess that could just be a carb adjustment.