I have a 2000 gas ford excursion limited 4x4 6,8 v 10 . I was driving pulling 2 1/2 tons and the motor started to make a exhaust sound at the top center just under the intake plastic.. The truck kept all power and drove home just fine . The exhaust sound increases with engine speed.. Can smell a little exhaust in cab . please advise,

2000 6.8  V- 10 4x4 Ford Excursion Limited  Gas .I was driving pulling 2 1/2
tons when the truck started to have an  exhaust sound coming from top center
of motor . Seems like it is coming from just under the plastic intake. The sound
increases with engine speed . I drove it home it drove just fine same power but
could smell a little exhaust.