Somewhere on my 2004 4.2L engine I have an exhaust leak. You can hear and smell it well when the engine is cold and under accelleration. After 5 min. or so of warm-up, the sound and smell subsides. I assume it is the expansion of something when it heats-up. Any pointers or ideas what to check 1st? If it would need a new exhaust manifold and/or gasket, what would a dealer typically charge for a repair like that? Thank You