I've noticed, especially since I've added a K&N air filter and Flowmaster exhaust that when my 2002 5.4L F150 (auto trans)switches from 1st to 2nd, it makes a squeaking noise. Sounds like a belt squeak, but I'm not sure. Anyone ever experienced this or know what it might be? It did it before, but not as noticeable. If I really get in it, it will also squeak going into 3rd.

 

Mine does too dammit! I have a 00 5.4 4X4. I just put 3in JBA cat-back. I have that same exact problem and it sound like its coming from the belt. I wonder if the belt and all the pulleys are gaining too much momentum? And/or Im wondering if there is a pulley that is robbing power because it doesn't want to turn fast, kinda like when the AC is running.