Changed oil and forget to put new in?
Just kidding. Check plug cross reference online till you find three sources that all agree. FLAPS men can be very unreliable.
FLAPS= Friendly Local Auto Parts Store. You are on the right track, don't take anything for granted. Could be detonation or pre-ignition for sure if you say no exhaust leak. If you have the old plugs, put them in for sure, that's a quick check. Did you compare the old to new plugs as far as physically?