don't worry. you're not the only one. mine does it, and it drives me insane. if you are near the truck it sounds like shit. when you crank it up by your friends, they say "dude, your truck sounds bad", and i'm like thanks, and then they are like "no, bad as in shit, not bad as in awesome." wow, that makes me feel so good. lol. I have heard they are pretty expensive, but i haven't priced one yet, i have a friend that works at a muffler shop, and he is gonna try to get me one. i would gut it, but because of the downstream o2 sensor, i am hesitant. anyone have any inpupt on a gutted cat on a taco?? anyways, your cat is prolly shot, also check your heat sheild. i rigged somethin up with a couple bolts and pipe clamps, puttin pressure on the bottom of the cat, that helped, but, i still had the rattle, so heat sheild made some of the noise, but most was from inside the cat. anyone else out their have any input on a gutted cat??? anyways, good luck.