I was getting another rattle sound and looked for about 8 hours total. I found the heat sheild under the mass air flow sensor vibrating against the exhaust header. The heat sheild is held inplace by three tangs. Two of the three tangs were broken. It is now being held by just the third one. If you have a GS300 check it out. My service advisor was suprised and said he had never seen it before, I find that hard to believe it appears to be a design flaw. It is located on the passenger side of the engine compartment. Follow the air duct to the engine and just prior to the engine you will stumble across a square box, under the box should be a heat shield, if not it has fallen off but the tangs that broke off will still be hanging there.