Well, it turned out to be loose bolts. Finally got the inspection port off, turned the engine and found one bolt was finger tight and another (out of three) was out about 1/4 inch. Removed them one at a time and replaced with some loctite.
Of course, to get it down, we had to drop the exhaust, which required disconnecting at the flange just before the muffler. Of course, the three studs were rusted and all three broke. Now I have to figure out how to get that back together. Why couldn't the geniuses at GM have routed that exhaust 3 inches to the rear?
Any thoughts?