I kept hearing this little nose out by the engine for months. Its was on and off so I really didn't go crazy looking for it. The other day when giving the truck a good cleaning (after driving on the beach) I found the problem. The forward soft splash shield on the driver side 2 of the top snaps / push fasteners were gone and the shield under certain condition's would flutter like crazy, Took 2 minutes to fix all is good. Always remember most times its the simplest thing