Yeah, that's NEVER a pleasant sound to hear! Not sure, but I would bet it's going to need a complete teardown, rebuild, and then clean out or replace the lines, the aux cooler (if equipped) and the radiator. You could always do what I and others have done, and spend about $350 or so and an afternoon in the junkyard, and convert it to a 5 speed! Hope 8t turns out to be a simpler fix, though. Good luck with it!