I have an 08 TrailBlazer I-6 with 74K miles. It's had a slight clackity-clack noise when cold started, but never a serious issue until today when I cold cranked it and my entire engine compartment sounds like a busted muffler. More odd, I have zero faults, and the holey-exhaust noise disappears after about 5 minutes of driving. All indicators point to a cracked exhaust manifold, yet the sound doesn't stay. Also, even after it's been running, if I shut it off and restart it, the sound returns until about 5 min of driving. Aside from the clackity-clack, it runs smooth. I'm not experiencing any acceleration issues, but a slight fuel smell when the noise is present. Any takers?