car pops through passenger side exhaust

[lqqknloh]

1988 mustang speed density. the car is stock it all of a sudden started to pop really loud through the passenger side exhaust when the car is hot or cold under acceleration.i replaced plugs, wires,cap,roter.i ran codes and got 11 and 67.any help would be apreciated since i drive my son 350 miles to the doctors next week.

 

[90stangLX302]

11 is "System OK, testing complete"
67 is "Neutral drive switch or circuit"< i have no clue what this means though

 

[lqqknloh]

well i dont think code 67 has any thing to do with it then. the car has 138000 on the odometer and has never been abused.i was thinking about switching the o2 sesors around and seeing if it pops on the drivers side.