Pontiac: 1968 Ponitac Firebird noise, pontiac firebird, ponitac firebird

Question
I have a 1968 Pontiac Firebird. When the car is COLD and I am driving and about to come to a stop the car will make a loud clunk. It seems to be coming from the engine compartment and I can feel it in the steering wheel. The clunk only happens just before the car stops and no other time. After I drive the car for 15 minutes and it warms ups the noise goes away for the rest of the day. Any ideas? The engine is a 350 V8 with a 2 speed polyglide automatic transmission. The tranny seems to shift smooth as well as the engine. So I do not suspect either of those two items. Thank you.

Answer
It sounds to me like it is the Powerglide trans downshifting back into first gear. This a common thing with a powerglide tranny. The reason that it clunks when it is cold and not hot is because the trans fluid is thicker when its cold and the seals are stiff. After it warns up the fluid thins out and the seals begin to soften making everything work more smoothly.
Try changing the trans fluid and filter. It should help. If you have a shop around you that can do a trans fluid flush, it would be better because you get all of the old fluid out and not just a few quarts.
Try that and let me know if it solves your problem.
I had a 69 Nova that did the same thing.