Before you try the premium fuel, I forgot to mention that the timing belt may be the cause of the noise. It has been known by many, including myself, to slap against the cover causing a marble type noise. The cause is a poorly engineered cover that does not allow clearance for the timing belt. I would start by putting a few washers between the cover and the engine, and if that eliminates the noise, you could then buy a new higher clearance cover for less than $15.00 from your local ford dealership.
