I just bough a 2007 is250, connected my music player to the AUX port and hearing a loud hiss and it goes louder when the engine is revving higher, any idea of where to start? thanks very much.

This is the solution

You're the man!

Brian7 is right. I use the PAC SNI1 3.5, also from Amazon, but a ground loop isolator is what you need. While you are waiting for it to arrive, you should also be able to eliminate the noise by running your phone or player off its own battery, and not powering it from the car.

I had this same problem in my 350 and did some research and found that little thing. Lexus actually had a TSB for it (ground loop isolator) but out of warranty they won't fix it. Luckily the fix is cheap