The Ford dealer service advisor said I had a "small exhaust leak" and referred me to a local muffler shop.
The muffler shop did a very thorough inspection and told me that several bolts on both exhaust manifolds were broken (V-8) and that Ford used cheaper bolts to hold them on. He said my frequent high altitude mountain driving caused prolonged heating of the manifolds, which accelerated the failure. He also said they use a higher grade of bolts than Ford when they put new manifolds on.
Their final word was that the clearances are tight and if they could not weld something onto a broken bolt they would have to pull the engine to get access.
The truck still has an extended warranty so I'm debating taking it back to the Ford dealer. Any thoughts?