Hey guys, I did a search in both the GMC and Chev forums, and didn't have much luck...
I have a 97 GMC Sierra Z71 with a 5.7L. At slow speedsduring braking and sometimes even in "park", I can hear a thumping noise coming from the transmission area. I have ruled out the common possibilities (U-joints, brakes, etc.) because it will do it when the tuck is stopped and in gear. I have had all the fluids changed and at the proper level (tranny, transfer case, and both front and rear differentials). Is my transmission or my transfer case on it's way out? It has to be something that is in motion when the truck isn't... I'm stumped! Any suggestions are greatly appreciated!

This is AFTER its stopped...not AS its stopping??

I'd say transfer case is most likely..is it the same in 2wd or 4wd??

Thanks for the reply Argon,
I notice the noise as it is stopping AND when it's stopped. For example, I can hear it as I slow down for a stop light, then the noise will continue after the truck is stopped while it's still in gear. It is louder as I am braking, but continues even when the truck is stopped. It doesn't happen all the time either. I usually start to notice it as the truck gets hotter, like after a long drive. I'm wondering if it's possibly the torque converter??

Good question about 2WD and 4WD. This is always in 2WD. I will try it in 4WD on my way home today, and see if I can hear it.