Its caused from detonation in the cylinder(s). Is there a loss in performance? Is your engine running hot? Check the timing? Keep putting high octane fuel in it until you find the problem. Detonation can ruin the engine.

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2005 FX4 OffRoad
4" suspension lift
3" body lift
Cat Back Magnaflow Exhaust
16" Pro Comp Extreme Alloy Wheels
315/75 R16 Kuhmo Road Venture A/T