The most common explanation will be the direct injection system that the ls460 and newer cars have versus the ls430. My understanding is the sound is caused by the 2 high pressure fuel pumps operating. There is also some type of solenoid(s) in play also, but I don't know the specific details. So this noise is normal.
The worst case scenario which I've read about on another forum is one owner who had the fuel pumps replaced under warranty, and the noises persisted. After some haggling, Lexus replaced all the pistons in his car and the noise stopped and has not resurfaced so far. This may have been piston slap, which is typically loudest when engine is cold right after start up. I believe piston slap does not necessitate corrective action
