Hello i recently bought a 2004 land rover discovery with 54k miles. i noticed that the tank said use supreme gasoline. i put 87 octane in it the first day i had it . Well it started making a knocking sound and i just started using the most expensive gas because someone told me thats what u have to use in landrover. It sounds like the knocking is coming from under the hood. has anyone else had this problem or know what this might be and how i can fix it . thanks