Instead of the pinch weld, the new heat shield is held in place using three gear tightened straps that surrounds the muffler - they look like huge radiator hose clamps. It refers to a type of mischievous gnome-like imp or demon, typically said to be around a foot tall, which probably has its roots in the old folklore of goblins and fairies. It involves cutting off the original heat shield and installing a new one of a different design. The origin of the modern term 'gremlin' is disputed, but is often said to derive from the Old English word greme, which means to vex or annoy.

Your description of hammering has also been used to describe this noise. This shield is pinch welded to an end seam on the muffler and during the expansion and contracting process the shield makes (what General Motors refers to as), a popping or snapping sound. The expansion of the heat shield takes place at a different rate that the muffler it's connected to. More specifically, the heat shield that surrounds the muffler is causing most of the grief. Your gremlin is heat expansion (and cooling contraction) of the exhaust system. The good news is that this symptom is very similar to an issue that other General Motors light trucks have had. Good try with your bud having a look around, but like any intermittent problem, it usually takes more than one attempt to find the source of this kind of thing. Intermittent hammering is only a good thing when you're building something Mel. This is driving me nuts Rob so I hope you can nail this problem.

Space gremlin noise