I replace my stock muffler with a Flowmaster original 40 series almost 5 years ago. It was LOUD.
At 70 MPH, about 2800 RPM, it was so loud in the cab that if I had a passenger in my truck
and we wanted to have a conversation, we had to almost scream at each other. That gets old in a hurry.
Even Flowmaster admits now "The 40 Series will generate extreme interior resonance within the vehicle."
Unfortunately they had no such warning when I purchased mine. In less than a year I exchanged it for a Magnaflow.
Better, but still a bit loud for my tastes. The in cab resonance however isn't to bad. I can live with it.
Will admit that most of the teenagers and 20 somethings told me they really liked the sound of the Flowmaster.
I'm almost 70, so that might have something to do with it.