Hey guys,

My wife's 2005 Silverado (5.3 Z-71) has just started making a clicking, scrapping sound under the cab. I've ruled out u-joints, exhaust leak, leaking t-case, and stuff stuck in the brakes.

Here's what happens: Under moderate to heavy acceleration, there is this clicking, almost a metallic scraping sound that goes away when letting off the gas or when it levels off in acceleration. I can say that it's very random, and sometimes it's very loud whereas other times it's very quiet.

When I switch to 4WD-H, however, there is no clicking coming from under the truck. This sound only happens under acceleration, none in neutral nor reverse.

I've tried to search high and low, but I need somewhere to start looking. Any help would be greatly appreciated! It has about 106,000 miles with no known rebuilds.