Everyone I’m new to this forum so let’s see how this goes I have a 2008 mustang GT when I’m picking up speed we’re going onto the freeway as soon as my car hits about above 3000 RPM I start hearing this thumping sound it’s back to back to back and it comes from the rear of the vehicle possibly the exhaust. I have long tube headers and Offroad X pipe and a straight piped rear. This problem is only started to occur recently and it seems to happen mainly whenever I first turn my car on at first I thought it was the spark plugs Because the sound is similar to whenever my spark plugs for Dad however I replace my spark plugs less than a month ago in the car doesn’t stutter whatsoever or hesitate the car has the same power and always has it’s just a weird thumping sound in the rear of the vehicle don’t know what’s causing it if it’s normal or if it’s Beth my car any info would be appreciated thank you. because the sound is similar to whenever my spark plugs were bad however I replace my spark plugs less than a month ago and the car doesn’t stutter whatsoever or hesitate the car has the same power always has it’s just a weird thumping sound in the rear of the vehicle don’t know what’s causing it if it’s normal or if it’s bad for my car any info would be appreciated thank you