Sound From Headers

[Fostang]

I talked my friend into getting a set of flowtech shorty headers for his 93 stang. He ended up picking up a set of their ceramic coated model. They look great on the car, runs and sounds a lot better. Only one problem when he goes into wide open throttle it makes a knocking noise. Sounds just like if you take a steel object a hit the exhaust. Only happens at wot. We have already checked the engine compartment to see if something is hitting againts it there is nothing on or around the header. What could this be???

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67'fastback- Edelbrock carb, intake, cam, valve springs. Road lifters, Comp Cam rocker arms, windage tray, cerama coated headers, dual flows with H-pipe, modded c-4, tci stall converter, perma cool electric fan, aluminum water pump, griffen aluminum radiator, accel 8.8 cable, wide cap distriburator, magnetic pick up, manley valves, v-matic hurst shifter, edelbrock performer nitrous, carter electric fuel pump, ignitor coil, solid motor mounts, magna racing suspension, poly urathane bushings, und