Alright - I have a 98 Silverado 1500 Ext cab Z71 truck. I have had it for a year and it has made this noise the whole time. As you accelerate it goes through 1-2-3 just fine and if you are light on the gas it will shift to 4th at 40mph. At this point it will make this rattling noise - almost like a chain slipping or rattling. It will continue to make the noise until about 50mph where it just goes away. The hotter the truck gets the longer the noise will last - up above 50 mph. It has not gotten worse in the year that I have had the truck. Here are a few of the suggestions I have had - A broken planetary gear, worn/loose chain in transfer case, rear end noise, u-joint, lock up solenoid with a loose wire that is cycling on and off with the vibrations of the transmission. If it were a broken planet it would have caused hell in my transmission by now, the chain makes sense from the way it sounds, but why only in the low end of 4th?? The noise sounds like it is coming from just left of center under back seat area - about where the transfer case is. I took it to a transmission shop, and they plugged in their computer and drove it around and told me everything was ok. To test if it is the transfer case, can I get a tailshaft from somewhere and just take the transfer case out? This truck has the towing package on it, so does that mean that I have the larger of the 2 available rear ends? Also - if this tranny is screwed I am going to put in a T56 - anyone know if the transfer case will plug into it?? Another oddity - in 4wd the inside rear wheel wants to hop in a turn - like the rear end is locked, but it is not slipping - is that normal?? Any help would be great - Thanks