I have experienced some rattle/whistle while traveling with speeds exceeding 200 km/h so near the top speed. It sounded kinda electric or like a loose welded medium link chain warbling. As the hood is made of alu and is shaking pretty much on high speeds, I thought that that will be it.
First, I rotated the hood bumpers half a turn counter clockwise to lift it a little, so it is tensioned more. It did not help much.

I degreased the hood striker and applied a thick coat of a gun grease (I used battle born, but I think that any really thick and viscose grease like red n tacky will do). You want it to be sticky, not slippery like HHS Lube in this case. I also applied some on the latch. Today I've had a ride with top speed and guess what? No more rattle!

 

Well, since you are 1 out of 1000 that will hit 125 MPH in an SUV I think we will see nobody else having this issue.

 

Yeah, in the US you usually adhere to the speed limits more or less. But Lexus sells like 100k RX a year so this post could help like 100 people yearly if just 1 out of 1000 drives that fast. :P
But the hood starts shaking well below 120MPH, so some of you may also experience it on lower speeds. Before going to the dealership you can just try this.

Just FYI: the RX is very stable while traveling 120MPH, I would say it is comparable to the IS200t.

 

Well I'll admit I've hit 100+ a time or two but just briefly to see how it handled in a safe low traffic situation. I did wonder how much more it had so in that sense I appreciate the info. If something starts to rattle at 120+ I would normally take that as a signal to slow down and not worry about what or why it is.

 

Well actually its top speed is exactly 207 km/h according to google maps and it shows like 220 on the speedometer... I mean... it won't disintegrate if you drive that fast. Of course I don't encourage you to break the law blah blah blah...