We have 1991 olds cutlass supreme... drove fine yesterday... put gas in it and then started back up. Sounded louder than usual, grumbling noise. rpms don't fluctuate, idles nicely, car rattles and vibrates at about 15-20mph and 40-49mph. Decreases speed normally on breaking, grumbles and chugs in reverse, but not idle issue. Doesn't slip gears, doesn't hesitate, doesn't surge... Can't figure out what it is... Any help greatly appreciated!

 

the catolic converter has a hole around the area its in probably from rust and when it is put in reverse the rpms drop farther than normal because the pre catalyst 02 sensor isnt reading a correct flow of exhaust because of the leak