I have an 09 Limited and have had this same problem since I bought the vehicle. You can see my post called "high pitch whine" to see if my issues match yours. So far the dealership has been unable to fix it. I have had a new radio installed, had the rear sub speaker replaced, and then had the entire electrical system analyzed to see what causes the noise.
Currently the dealership has discovered that for some reason, and they dont know why, the radio is getting too much power (or putting it out) and is causes feedback. I am waiting for them to get in another radio to replace my "newest" one.
They finally determined that it was the radio itself causing the problem because they took mine out and installed a Nav system radio ( I only have the base radio without Nav), and as soon as that radio was installed the noise went away. So, perhaps I somehow got a defective radio that was then replaced by another defective radio.....
So, I am hopeful that a new system will be able to eliminate the noise.