Rattle noise

For the last few years, I been hearing some sort of rattle coming from the front of the truck. You can hear more so from the drivers side. So I began trying to figure out where it was coming from. This rattle comes and goes. I can hear it when I'm revving and around 20-30mph. I was reading up on it and the conclusion that I came up with was, maybe the cats might be going bad. I contacted my local Toyota dealer and the insisted I take it in for them to inspect it. I took it in, in hopes they would figure what that noise is. They concluded that the cats were fine. They did tell me that the techs did hear some noise but could not find the source of it and that my truck was running fine. At this point I'm pretty stump on what it can be. Anyone with similar experiences or knowledge can chime in