theres your problem (IMO). you dont need the platinums in spark plugs yet, they are probably burning too hot. all you need is a cooler plug, some copper ones, and your detonation issue should go away. if it doesnt then its probably a fuel/air delivery issue. im 95% sure its your spark plugs

EDIT: no COOLER plug... just a copper plug (wtf was i thinking??)

 

i cant type for my life today.. whats probably happening is your platinum plug is retaining too much heat and igniting the mix too early. the higher octane fuel resists this, thats why it went away. get some copper plugs you can run 87.

as for the rattling.. im still trying to figure this one out.

 

I'm confused as I just looked up at Motorcraft website and this is what I bought:

MotorCraft AGSF32FM (PLATINUM FINEWIRE - NICKEL PLATED SHELL).

I bought them at Autozone and that was one of my choices amongs Autocraft & Bosch. My Chiltons manual said AWSF32PP (Platinum Plus) which I could not find.

Wow - how do I cross reference to a copper wire plug?

 

STAY AWAY FROM BOSCH!! thats rule number 1 lol.. trust me mustangs have nothing but problems with them. just go into the auto parts store and ask them for copper plugs.i dont know if they were OEM plugs, but they should have a variety as ford used alot of the same design for plugs in a wide variety of ford vehicles.