Ok here is my reasoning and conclusions. I have an 01 Bullitt #1307 Black in color. Everything is stock except for some visual upgrades (wheels, lowering kit, rear bumper cover, headlights, grill, hood etc.....). I just installed a Mac o/r H 2 1/2 and 3" Flowpath catback with 4" tips. Amazing sound on startup and cruising. Very loud and deep, almost the perfect sound combination except for one thing!!!!! Once your reach 2100-2500 rpm the exhaust starts to sound like straight pipes making that machine gun note. Like on a backwoods chevy or something (I know right?). First i said ok it must be an exhaust leak somewhere. Went back and put some torque to it. Not a lot of difference. Ok, let's go check it again, back on the lift and cranked it all down as snug as it gets. Still making the annoying sound like if you put your lips together and blow air out, except it is the exhaust tone sounding like a truck. Now I'm waiting a few days for the system to warm up and cool down over and over a few times then back on the lift to tighten again. Another thought? Maybe it's the 2 1/2 off road h-pipe flowing into the 3" catback system. Maybe the catback system is acting like an echo chamber from the smaller pipe??? After some more research on 2001 Bullitt exhaust I know that ford engineers specially tuned the exhaust on the Bullitt to mimic that of the fastback mustang in the movie. Well actually it does sound like that but amplified due to the opened up system on the car now. My research has shown that Ford engineers specially tuned the Bullitt exhaust using sound management/recognition systems software and computers to tune and develop the factory exhaust system to mimic that of the fastback by opening up the exhaust flow and placing different mufflers and tips on the car! I started to wonder if it was my 4" tips, but then remembered it did the same thing before they where installed with no tips at all. So....one of my questions is this! Could Ford have somehow used the onboard computer to aid in the sound of the Bullitt? If so then a programer SCT 3 for example should be able to replace this program with one for the current system and fix the issue. Could it be ignition inside the exhaust pipes themselves causing this sound issue? Again would a new Breenspeed tune for example fix it??? I have a drastic loss of fuel milage and my exhaust is killing me???? In part the confusion comes from my last 00 mustang which ran the exact same exhaust setup and sounded like a beast. No issues at all. It did however have a chip, CAI, MAF, Throttle Body. Unlike my current one. Anyone have the same issue and resolve it keeping the system?