I have done extensive research on this for about a month straight, so bear with me. I have a 99 gt that ticks on idle, increases speed with rpm. Goes away when I'm on the throttle. I have replaced the lash adjusters, valve Springs, coil packs, plugs, checked for exhaust leaks. It's in the number 5 cylinder. When I disconnect the electrical to said cylinder it stops so that is about as far as I've narrowed it down to. Car runs great, no loss of power, just embarrassing! Mods are just bolt ons. CAI, 75mm tb, BBK equal lengths, catted x, slp cat back, and running with base sct tune. Always 91 octane and recommended oil weight. I know lots of SN95 owners experience the same. Just pulling my hair out. Have owned mustangs of nearly every generation and this is a first.