Any drivetrain experts out there?

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

Sounds like torque converter clutch shudder. At 40 it starts to lock up the converter. I am not sure at what speed it should be completely lockup but eventually it locks up and the noise stops.

The problem could be in the converter or with the tcc apply pressure (valve body).

Here is how to verify if this is the problem. When the noise goes away (say over 50), keep you foot on the gas and at the same time lightly tap the brake for a second. This will cause the converter to unlock. When the brake is release, the converter will again lockup. If the noise comes back you know what it is.

//2000CAYukon

I agree with Yukon on the shudder at 40mph, although I am not that well-versed in all the details of the locking converter.

For the rear inside wheel-hopping, I would suspect you have a limited slip rear diff, which may be locking up a little more under the added stress of 4wd, as it takes more power to make the truck turn with 4wd on. The diffs are really working hard with 4wd on in a full-lock turn. Every 4wd I have ever driven feels like it's struggling a little in that situation.

Could also be a bad planetary gear/bearing in the trans, this will have similiar symptoms.