Rattling noise on 2003 RX 300 at cold start only?

Do you run a thicker oil in the car now that it has 128K miles on it ? First check for any codes and check the oil in the A/C compressor. At the next oil change, try running a thinner oil in the engine. I have had this happen before were the new oil is too "thick" (viscosity and/or temperature wise), so it takes a while for the old worn oil pump to get the "new" oil thru the system. If it was a heat shield rattle it would not go away until the exhaust was heated up after driving for a few minutes. The fact that the noise goes away so quickly, leads me to believe it is "Lack of lubrication based noise" and that would lead me to check "engine oil, steering pump oil, A/C Compressor oil" levels and viscosity.

sounds like gasoline is laying awash the cylinders...you know the fuel rail will pressurize whenever the "on" position is on~ if you start it immediately with going from the 'off' position directly to the 'start' position it happens this way too? Thinkin' there is too much gas on startup and takes a minute for oil pressure to start lubricatin'....pull the oil stick and see does it smell of gas? that would seem the way of things~

Sounds to me like lack of lubrication on top end until the oil pump 'catches up' Too much oil? No. tell me you don't have a Fram on your car, Fram filters restrict oil flow with the cardboard filter element they use.

your "too much oil" is a good guess...that is gasoline raising the level...we have to find out which component is distributing too much pressure...perhaps the fuel pressure regulator ain't regulatin'~

Thanks for all your answers, gives me a good place to start. Bought the car a month ago and they did the last oil change so I'll have to make sure they use the correct oil, wouldn't doubt they threw 10-40 in there from the big barrel. My mechanic could not find anything wrong when he inspected at the time of purchase, but that was mainly visual inspection. I'll see if there is a gas smell, my "too much oil" comment is because I have a hard time getting a level on the dip stick, seems like oil gets smeared up and down the stick with no clear mark. Car is due for an oil change so I will make sure to bring all these up to my mechanic then. Again thanks for all your feedback, first time here and very impressed.

you know you could pull the fuel relay while it's runnin' then--- try to start it to see if it IS awash with gasoline....just another diagnostic tip from your friend judge_roy~ (then replace the relay)~

If you have a dipstick for your car without any marks on it for a "proper" level indicator, then go buy a new dipstick for the vehicle from a dealer or a local auto parts store (NAPA, O'Rielly, AutoZone, etc.). The cost is cheap and the question of "Is the oil level too low or too high" will never come up again.