On my L99 auto, I did a delete first and it wasn't too loud at first. But, the constant 'drone' of the exhaust on the highway was a bit much for me. I ended up w/ a Walker BTM muffler that is a straight thru design, but cancels out some of the noise. To me, I don't think there is much diff between straight pipe, or muff w/ straight thru design other than noise level. W/ the BTM, I noticed a small gain in mpg's. (I also did the 6637 mod at the same time, which probably helps too.) As w/ anything else, everyone and every truck is diff, so I would suggest a straight pipe first since it's cheaper than a muffler; if you don't like it then spend the money on a muffler. Just my opinion tho.