My 04 LS430 is making the exact same noise. It is definitely A/C related, because if you start the car without the A/C on (note: having the climate controls on 'Auto' is the same thing as having the A/C on) - you will not hear the sound - but you will hear the sound when you turn on the A/C or press the 'Auto' button.
I also would like to fix this problem but I am not willing to replace a bunch of crap that isn't broke to fix it.
Anybody else have fix to this problem that specifically addresses this problem?
Thanks
FoF