I do not want to say that I’m sure for 100%, but I think what you hearing is not an engine sound. I guess this is a typical jeep’s drivetrain sound. Jeep’s transmission noticeable noisy from the 1-2-3rd gear until it shifts to 4th gear or you gain speed up to 50mph. This is a normal sound, but it is very annoying sound. The only way to suppress this sound is to apply the sound attenuation mat, “Accumat” or “Dynamat” or whatever is available. Pit it directly to the floor sheet metal, and/or wrapping up the transmission tunnel. Just my crazy suggestion.