I read through this thread and thought I'd offer one more source of potential noise. That source would be a partially failing alternator. Not sure how many people know that a diode on a modern alternator can fail and no light on the dash will show this - you are still partially charging. Only way to detect it* is through looking at the output on an O-scope or similar. What you would see is a partially rectified output with a much less 'smooth' line to the DC being sent to the battery and any other load devices. Among other ramification, this can create issues through fluctuating voltage for things like a display plugged into the cigarette lighter.

* Note: Well I guess if you were previously barely able to run something with a healthy alternator, and a diode failed you might notice, but in that scenario you're already probably replacing alternators on a regular basis due to load induced issues.