About a year ago my truck developed a buzzing noise coming from the drivers side when I was driving on the hwy, @also the low tire pressure light came on at the same time. I had the sensors replaced. The problem would come and go so I took it to a muffler shop and they said that the drivers side cat was going out. But then the noise stopped until about a week ago, so I replaced the d side cat and that did not cure the noise, had all heat shield checked. no loose clamps or anything, exaust was fine the shop said. But the noise comes and goes at certain steady RPM, and stopes when i let up on the gas. Any suggestions, just a note I do not have any codes that displayed.