just bought a '03 scab fx4 with 5.4. it has a true dual from cats back. had a generic flowmaster di/do . I think it was the same as a 40. had a terrible drone at 1800 rpm. I had a 14 inch magnaflow di/do installed and still have the drone at 1800 - 2000 rpm. the pipes are 2.5 inch from cats to muffler and out of the muffler. they are run dual out to the side behind the rear tire. truck also sounds like it has a miss at a certain rpm in overdrive,( I know it isn't missing) like the exhaust is "rolling" . it isn't a "clean" sound. I was thinking of having a couple of small bullet style mufflers installed to act as resonators . I really don't want to spend a whole lot of coin on this . maybe taking the single muffler out and putting in two separate mufflers. or downsizing to 2.25 pipe. any advise would be great.

 

If you look at true factory duals like on my Mustang there is a crossover before the muffs. If you are running true duals from each side into one muffler then two tailpipes out, it's going to drone. Is there an X-pipe or H pipe after the cats and then to the muffler? If not I would think two separate muffs would help greatly with drone.This is the #1 reason I leave my truck stock. That drone will drive you crazy. Dynomax has a muffler with CRF technology that is supposed to eliminate drone. Maybe read up on it?? IIRC some of those Flowmasters are awful for drone....as is Magnaflow.....too much resonance in that small of a muff.

 

the magnaflow has a "x" pipe in it. I bought the truck with the duals already on ( from my brother who has different ideas about things than I do). I would have done things differently if I had been monkeying with the exhaust. I was thinking going with a 2.25 inch pipe to the muffler instead of the 2.50 , to see if that would slow down the exhaust and stop the "rolling" sound and some of the drone

 

update on exhaust. the pipes coming from the cats are 2 1/4 not 2 1/2. so that being said . I had 2 inch from cats back to muffler. this seemed to help with the rolling sound but still have a slight drone. when I save up enough cash to run the pipes out the back instead of the side i'll prob go with 2 inch instead of 2 1/4. truck also seems like it has more torque.