In December 2003' I had a Magnaflow SI/SO Muffler put on my 2003 5.4. I just went with the new muffler and 2.5 inch pipe out to a 3" rolled stainless tip. I love the sound and I think I gained a small amount of HP and Torque through the entire RPM range. I was recently changing my oil and while under the truck I noticed a spot of black soot on the weld where the tailpipe meets the pipe exiting the muffler. I had a friend start the truck with me under it and I could not feel a leak or hear a leak at the spot of the soot. Could it be that the leak is just too small for me to notice. Should I take it back to the shop, I have a warranty on the labor and parts. Or should I just leave it alone and not worry???