the o2 sensor could be knocking I didnt mod my firewall and can feel it hitting but only in reverse the engine is going to move a little so it could be something just lose rattling around at certain ranges inclines and so fourth
when mines hitting I feel it in the steering wheel seems like if the gas pedal is vibrating its more likely to be from your DP and lower pipe area i would check the clearance there
does it smell like burning plastic?
have you checked your o2 sensor wire and plug wires for heat dmg?
