My 2004 LS 430 makes a frequent fluttering noise. It sometimes stops by itself, or sometimes when I am stopped and move the shifter. It sounds like it is coming from beneath me, not the engine compartment.

 

I have had a similar fluttering noise on almost Lexus vehicles. I think I read somewhere that it's the air circulation doing it. There has to be a thread somewhere that describes it better than I am, but I can't find it.