Whether you have satnav or not, if i'm not mistaken your car should still have the 3.5mm auxiliary jack in the armrest, this allows you to use the bluetooth A2DP gateway.
That's a known ground loop issue with some is250's and 220d's. Lexus published a fix that is free under warranty. if out of warranty, buy yourself a Ground Loop Isolator very cheap on eBay, that should solve the problem.
Is there a walthrough on how to do that? As mounting a ground loop isolator usually deteriorate the sound quality.