I experienced this exact problem, did a search on the net and found the following:

Why does Lexus RX350 idle very rough in gear and standing still for the first 3 minutes or so after sitting all night?

No answer was given. Anybody know why or have this problem?

 

How rough idle?

 

Rougher than normal, sounds like an older car. It goes away and becomes quiet after a little while. It's obviously computer-controlled, but it happens only after it sits overnight.

 

I take that when you say rough means higher rpm and louder than normal. I'm not a car expert, but all my cars idle at higher rpm and louder when cold. Even just leaving the car overnight can cool down the engine enough to have a cold start. As long as the engine returns to normal idling rpm after a few minutes, I wouldn't worry about it.

 

It doesn't happen when the engine is running (in park), but when it's in gear. It's quiet when in P and becomes loud when shifted to D. I'm in California and it doesn't get cold at night.

 

Next time when you drive.. start it.. put it D then record the idle... for about 2-3 mins.. See what kind of results you get

I'll try it tomorrow...

When you haven't driven the car for about 5 hrs or something or is has been sitting over night- the car is cold (engine is not warmed up). The car usually has a higher idle than when its warm..

 

Do you have the AC on?

 

Yes, it's usually on. Is that the culprit? My wife took the car this morning so I wasn't able to check it.

 

I noticed mine does have some rough idling and noise when my AC is on and I'm just about to drive out of the garage. It only happens though when I'm making turns. I thought the noise was from the steering. I never really paid too much attention to it afterwards because like you said, it just disappears when you are already driving. I also normally turn on the stereo the same time with the AC so I don't really notice the noise anymore. I will try to pay attention to it again.

 

32kcolors, I just checked mine today and it seems that the noise I was referring to got nothing to do with the engine but rather with the power steering when turned fully to the right or left and held in that position for a while. I believe that's normal for all cars and actually should not be done so as not to damage the power steering.
I did notice that rpm is higher on cold start, but I don't hear or feel anything out of the ordinary to be a concern at all. As long as your high idling is steady and not erratic, and then the idling gets smoother when the engine gets warmer, you should not be concerned.

 

I've also tried it.. started my car, turn on the AC, put it in Drive.

Idle dropped to about 1000 rpm.... But no problems.

 

Your profile indicates you have a 2007 model.

Has the dealer looked at it yet?

Warranty should fix this.