I've been having this "ticking" sound problem for a while now and when I took it to the dealer about a month ago, I was told that I needed to use premium fuel. After using premium for a month and the noise still didn't go away, I went back to them and was told that now Lexus is aware of the problem and is coming up with a fix soon!
My comment to the dealer was that "if I wanted my car to sound like a diesel, I would have bought a diesel." So I guess we just need to hang on and see what happens next!
