Did it start right after you put the mufflers on? If so it was probably always there and you just didn’t hear it through the stock exhaust.
If not change your fuel filter. It only takes 1 bad tank of gas to clog one up and it cases lean conditions (especially under “surprise” high load situations). They are cheap and easy.
Popping or backfiring are lean conditions 99% of the time.
Waiting on 2019 GT500 2012 GT Vert Brembo - Stock-ish 2012 F-150 FX4 EcoBoost 4x4 406rwhp/522rwtq. The most powerful EB F-150 on the planet?
2007 GT-TSR/Stops/Turns/Custom Everything/3.4L Whipple Crusher/1000hp SOLD