Possible Causes:

WARNING: This page does not describe the selected car, but rather 8 other vehicles, including the 2006 Ford RV Cutaway, 2006 Ford Pickup, 2006 Ford F550 Super Duty, 2006 Ford F450 Super Duty, and 2006 Ford Econoline. However, it is still accessible from the selected car via links, so may be relevant.

Snow, plastic bags, or other foreign material may restrict airflow at the air inlet.
Misrouted air cleaner ducting.
On engines recently repaired, rags or cap plugs may have been inadvertently left in an air inlet pipe.