Hmmm, still need to pinpoint that rumble, make sure it's not a bearing pack in the A/C going out. But it could be because the idle's loping because it's not advancing the throttle properly as I stated above. The IAC is located on top of your intake to the passenger side of the throttle bore itself, just in front of the butterfly plate if you take the intake tubes off. Cleaning it with TB cleaner and a soft brush *gently* will usually free them up. Clean and blast the bore it goes into clean with more TB cleaner and a soft brush, an old child's size nylon bristle tooth-brush works great here. Srub and blast all of the brown/black crud you *will* see on both the valve pintle and the bore it goes into until it's shiny metal. Mine used to lope and bump around badly when the A/C was first engaged when I first dragged the truck home. Cleaning the throttle up and the IAC valve got her back into shape. If it doesn't, well then it's time to look at other possibilities; as in "When was the last time you had it tuned up?" Etc.etc.etc.