I HAVE HAD THE SAME THING HAPPEN BUT MINE IS ON A 4.6 IT SOUNDS LIKE THE MOTOR IS ALMOST GOT A MAIN BEARING BAD AT TIMES. A BUDDY OF MINE SAID THAT IN THESE MOTORS THERE ARE HYDRAULIC TIMING CHAIN TENSIONERS THAT WHEN THE RPMS REMAIN LOW FOR A WHILE THE CHAINS SLAP. HE SAID IT IS SOMETHING YOU JUST HAVE TO LIVE WITH. IT SUCKS AND HAS PEOPLE ASKING QUESTIONS LIKE, IS YOUR MOTORS KNOCKING