Originally Posted by Sh0velMan

What you are hearing is the pipes themselves vibrating.

The exhaust has more pipe and longer straight runs, this makes for a lower resonate frequency as compared to the stock Y Pipe setup.

You can hear it inside the car because the vibrations are transmitted into the chassis because of how close in proximity the exhaust is and people can hear it when you are right next to them, but that is it.

I've explained this before in another thread, basically it isn't sound being emitted by the exhaust gasses making the sound, it's just the organ pipes under the car vibrating.

So yeah, the car will sound different on the inside, and if you are pulling away from someone who is real close to you, they'll be able to hear it for the first ~20 feet or so, but after that, you're silent as a ghost. It's also just a deep drone, it isn't a real "exhausty" sound.