I agree with the horrible drone of the Flowmaster. I had one on my truck for literally hours. I went with straight pipe...still one of the best mods I ever did.
BTW...here's a copy/paste of what I wrote back in 2006 (and reposted last year) regarding my muffler testing (I tested a bunch) and my decision on doing the straight pipe with my 2006 6spd:
"Well, I did it today...
Straight pipes here I came!!!! I put the Flowmaster Super40 on Friday, and I just needed a little more obnoxiousness. The muffer shop said, "That's not loud enough for you?" when I brought the truck in. I told them I can really only hear it when I nail it but I just wanted more midrange "honk" and raspiness (what most people try to get RID of ha ha).
Well, needless to say, I got the exact sound I was looking for.
Anyway....the sound is PERFECT. It is not NEARLY as loud as you would think being wide open. Sound at idle and slow cruising is actually quieter than the Flowmaster Super40. When you nail it, it sounds like a true 60's musclecar. It just SLAMS without being too obnoxious. Its definitely not "Bubba-esque" because it remains reasonably quiet until you nail it. Everyone at the muffler shop (customers included) were full of grins and cheers. I am VERY happy with the results! I'm telling you, you would not BELIEVE how well the truck sounds and performs with no muffler. I, myself, didn't believe it...until I tried it! And I paid quite a lot of money to get the Flowmaster and install it. I wish I would have ran it straight in the first place! A lot of people here advised me to do that...but I was scared it was going to be WAY too loud. Trust me...it ISN'T.
The muffler shop let me audition several mufflers, BTW. Tried Magnaflow Wide Opens, various Flowmasters, a M something or other race muffler (sounded nearly identical to straight pipes but LOUDER...didn't like that too much). As far as a MUFFLER, the Super40 DEFINITELY is the best sounding. It has that Flowmaster grunt, but amplifies the actual lower midrange sound moreso than the Flowmaster 40, which resonates the super low frequencies MUCH more than no muffler or even the Super40 while quieting the upper raspiness. The regular 40 got rid of too much of the rasp that I was looking for and created way too much low resonance inside. The Super40 definitely has LESS interior resonance and less booming drone. If I were to put a muffler back on, I'd go again with the Super 40. Yes, it is a tad soft, but not nearly as bad as the others. The Magneflow Wide Open changed the tone too much...just didn't sound "right"...some frequencies cancelled out, and it was farty and too poppy. It also made something behind my seat make a buzzing noise.
So, the bottom line...do the single straight pipe if you want midrange thickness and slamming tone like a 60's musclecar which is ONLY obnoxious when you WANT it to be (depending on how much throttle you give it).
Hope this helps anyone who is contemplating what I did."
Bottom line...the Flowmaster was LOUDER inside than stock (and the drone drove me nuts in a matter of minutes) and much too subdued outside. Straight pipe was...and is...perfect for this truck.
Here's a link to that thread from last year which I will also bump:
I've actually since went with straight true duals on a whim; I have the stock cats but after that I have two separate 2.5" pipes with a small H pipe as close to the engine as possible. There are no mufflers. The odds of getting a power increase in a certain powerband are a complete crapshoot doing something like this, but man oh MAN did it make a difference. I spoke with Mike in Az before doing this, and we both agreed there was no real way to tell what would happen without trying it. I was TOTALLY happy with the stock setup with a Y-pipe, but I always wanted to try true duals since I use my truck to pull my Airstream and am always looking for more low end torque and pulling power. My GOD did it make a difference...it's not insignificant at all. The straight true duals ARE louder than the straight Y-pipe, and slightly raspier at first. It could be slightly TOO loud for alot of people just because of the nature of the tone. The straight Y-pipe's tone is PERFECT with just the right amount of volume, rumble, and canon-fire when you step on it. Straight true duals is actually higher pitched than the straight Y-pipe (even with an H pipe crossover which is supposed to be deeper than an X pipe which is actually supposed to be higher pitched). True dual straight pipes STILL sound GREAT, and again drone is NOT a problem; it seems that the mufflers on these trucks actually CAUSE the drone more than straight pipe. But, sound-wise, straight Y-pipe is much less obnoxious and more pleasing than straight true duals. But, the real world towing power difference up mountain climbs and on take-off with true dual straight pipe is unreal and TOTALLY worth it. Without the trailer, with straight true duals the truck performs as if it weighs 2000lbs less; it's simply amazing how it takes off with nothing but a tune, the true dual straight pipes and 4.56 gears. My truck equipped the exact same way but with the previous straight Y-pipe was amazingly quick, but now it's SCARY fast for the amount of weight and mass it's moving. I'm very happy. Cost was about $400.
Hope this helps!