I have a buzz/hum in my stereo too. It occurred even after swapping out the head unit so I assume, as others have said, that it's in the feed to the Infinity amplifier under the driver's seat. I haven't cured it yet but putting a cap on the power input to the amp sounds like a great idea. Thanks for the tip.

Weird thing about mine is that it comes and goes. I can get it to go away temporarily by cycling power on the head unit or by hitting the dash. It usually comes back at the next bump or within 30-40 seconds. My aging ears are less sensitive to the noise but it drives my daughters nuts.