Check the vehicle with the dealer you bought it to determine the method of shipment. If it was shipped via an open car freight train it is rust particles from the trains braking system. The dealer might call you nuts but have them call their regional service manager and he/she should know (if they have been doing the job long enough). You can have the hood repainted or the body shop can use a product from Zep something kinda like an industrial fallout cleaner. Trust me our new Mazda had the same problem and now is good as new.
Tim
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"The tree of liberty must be refreshed from time to time with the blood of patriots and tyrannts."
Thomas Jefferson