Hello all, and happy holidays! I could use some help here please; I recently purchased a 2003 Yukon XL with 140k on the clock, 5.3l engine. I was a little rushed after looking at many Tahoes/Yukons, especially since my wife was due with our 3rd child in less than 2 weeks. I made one sinful mistake in my purchase... When I went to look at the Yukon it was warm already, supposedly because they needed to put gas in it. All checked out; very very clean, etc. I got cash and picked it up about 4 hours later and started it again somewhat "cold". Again, all was fine. After sitting for a few days at my house, I started it again. This time I noticed a hollow knock sound coming from the exhaust. Stock exhaust mind you. I have another 5.3l truck that does not make this sound. When you first start it cold, it smells a bit of unburnt fuel, though only for a minute; could be normal. The knock eventually goes away, but it takes a good 10 min. It's a hollow sounding knock, but again, you CANNOT year it in the engine compartment or in the wheel wells or under the truck, only from the exhaust. It doesn't get louder if I rev the engine... might even go away when I do in fact. Oil was changed by last owner about 2k miles ago with conventional. Oh, it doesn't take long to turn over, but it doesn't spring to life either the way my other 5.3l does. Thoughts? I can't seem to nail this one down with a Google search. Thanks all!