Hello and welcome to JustAnswer!
The problem may be in the power steering motor, which is part of the steering column. When an electric motor starts to fail it can cause electrical interference and noise and cause problems such as you are having.
It may also be a poor ground to the power steering motor or the radio or amp. It would be a good idea to run a redundant ground to each of the components and see if that has fan effect on the noise.
An RF filter installed in the antenna would help the issue also.
You mentioned wanting to know where you can get assistance with this, if it's not something you're wanting to repair yourself then the best place to go would be a Nissan dealer.
Thanks for reply but not much help at this point. Have you heard of this problem before? The tone is in the 400Hz range (guessing) which makes me think it is power supply related (as with a pulse width modulated type of motor/servo control).
I assume that the sound system is not at fault since FM, CD, USB and audio input are all clear as a bell.
The dealer checked ground, etc. and said nothing else they could do. I have until Jan 2014 to get this fixed under the original warranty.
I demonstrated to the shop manager the fact that a distant station (600AM) is perfectly clear until the steering unit is energized. Then the tone overwhelms the station's audio. A tech checked it for bad motor, poor ground, etc. and the manager said it now might be somewhat better (it's not) but that's all that can be done - I'll have to put up with it.
Your suggestion of a redundant ground makes good sense. What do you think my chances are of returning to the dealer to get this resolved? I'm getting too old to be crawling under the dashboard - and it's still under warranty until January.