It’s always been a little noisy at startup, has gotten worse lately. It usually goes away as the car warms up.

 

Take your RX350 to a mechanic. All we forum members can do is guess to what it "may" be.

The best solution to your problem is to have an mechanic do an actual in-person inspection of your engine; maybe two different repair shops to be certain that both shops recommend the same repair.

 

If possible, take to a Japanese Specialty Shop if you don’t want to go to your local dealership.


EDIT:

It would be helpful if you gave more background info like how long is “always”?

How long is “lately”?

And how long has it “usually goes away as the car warms up”?

 

How many miles are on it?
The noise often called the death rattle at startup has been discussed many times and is usually blamed on a few different sources among them the VVT valves- filters, oil filter, not changing oil enough etc.
See these prior threads:
​​​​​​https://www.clublexus.com/forums/hyb...up-rx450h.html
​​​​​​https://www.clublexus.com/forums/hyb...cold-soak.html
​​​​​​https://www.clublexus.com/forums/rx-...ine-knock.html
​​​​​​https://www.clublexus.com/forums/rx-...d-startup.html

 

Sounds like something serpentine belt related.
Remove the belt, start the engine and go from there. You’ll be able to rule out or confirm if it’s the water pump, tensioner/idler, alternator.