I have a rattle, almost a buzzing sound, that comes from under my pick up truck sometimes. I hear it most often when I am pulling my boat when just pulling off from a stop. It drove me crazy! I finally found what was doing it. Its the catalytic converter! I had to replace a piece of my exhaust pipe and I told the guy that was working on it about the noise. He said," Lets see if it is the "cat" doing it". With the pipe off behind the catalytic converter you could look into the end of it. He shook the converter and sure enough it made the rattle sound. It turns out that some catalytic converters are made with material that shrinks a little after they have been used a few years and that lets the "guts" move around inside and rattle when the engine is under load like when I pull my boat or punch the gas to get out in traffic in a hurry. I took a hammer and knocked a small dent in the top, bottom and both sides of the catalytic converter's housing and put a stop to the racket! That beats the crap out of the alternatives, listening to it, paying several hundred bucks to replace it with one that will probably do the same thing or just taking it off and putting a piece of pipe in its place which happens to be illegal. The guy that was working on it said that he wasn't touching it but would make up the piece of pipe to replace it if I wanted to do it myself. I figured why bother as long as a few little dents would stop the aggravation.