It is hard to say what the problem is without having the car in front of me. It could be a collapsed lifter, or exhaust leak, or detonation or any number of other problems. Does it reproduce if you go to 75%-WOT with the car parked or does it only happen when the car is moving? A video (with audio) would help a lot.
'98 3.8L Convertible (Red w/ White top)
’07 Shelby GT500 (White w/ Blue racing stripes)
YouTube Channel: Pet Rock’s Garage