I have a 92 Full size Blazer 350 5.7 with auto trans that I get what sounds like a chattering steel only when in drive and moving. Get's worse the more I drive it as it heats up. When I get the sound while driving and throw it in neutral it goes away instantly. Have heard different opinions about what it may be but I do not get it at start up and nothing at idle. Transmission shifts perfectly and has about 110k. Just changed the pan gasket and filter problem still exists. Someone mentioned a possible flex plate issue. Others said this is a common issue... Any ideas?

 

U-joints? Could be the catylitic cover/heat deflector?

 

It's something in the drivetrain, The heat shield's do rattle but that is not the current problem.

 

NOT U-Joints then? Output shaft on transfer case? Yoke bearing on rear end? I DO know on my '94 Yukon the electric switches went in the tranny. Per the dealer mechanic that lived next door after about 60k your living on borrowed time with those. But I don't think that would make your noise.