Hi Ray O, you were right:

Well, I took my ES350 to the dealer for the 10,000 mile scheduled service
and told them about the ticking sound heard when the engine was cold and
when accelerating or driving under load - like up a hill.  The diagnostic
maintenance guy got in the car with me and we drove around the area, but the
engine was warmed up and I could barely detect the noise.

We returned to the dealer and he got an identical car off the lot and we
started the drive out of the dealers lot.  He heard the ping (tick) sound
immediately.

He told me that the Lexus engineers had cut part of the skirt off the
pistons (I guess compared to the Avalon 3.5 L), resulting in a lighter
piston. He said that the sound we heard was "piston slap" and offered no
excuse or remedy!

My advise to all future Lexus 350 buyers is don't buy it.  Instead, buy the
Avalon.  It has the unmessed-up 3.5 liter engine and runs on recommended
regular gasoline.  The car is quieter, has reclining rear seats, and I think
they even put in the same 6-speed tranny.

Graybeard