Here is a direct quote from the 04 Navigator owner's manual:

Octane recommendations
Your vehicle is designed to use “Premium” unleaded gasoline with an (R+M)/2 octane rating of 91 or higher for optimum performance. The use of gasolines with lower octane ratings may degrade performance. The use of gasolines labeled as “Premium” in high altitude areas that are sold with octane ratings of less than 91 is not recommended.

Do not be concerned if your engine sometimes knocks lightly. However, if it knocks heavily under most driving conditions while you are using fuel with the recommended octane rating, see your dealer or a qualified service technician to prevent any engine damage.