Ford F-150: Exhaust Manifold Problem

1999 Ford F-150 XL
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Q--I have a Ford F-150 4x4 with 150,000 miles. In the last few weeks every time I start it, it sounds like there is no muffler. After I drive it for awhile it sounds fine like it always has, but if it sits for 15 minutes or more it goes back to sounding really loud again. It runs fine otherwise and I had it on the computer a few weeks ago and it didn't show anything.  What might I be looking at?

A--I suspect a problem with the exhaust manifold. On a cold engine the manifold may have a gap between itself and the cylinder head. As it gets hot and expands, the gap closes and quiets down. Pop the hood, start the engine and listen for the source.