Any correlation with cell phone use (either a call or SMS, etc.)? Just a thought, but I'm able to tell when a call or message is coming in
before the phone rings. I can hear the phone "talking" to the cell to set up the ring. This happens while I'm on a land line phone or with the amplified speakers on my PC, etc.
Something has to be periodic about what you're encountering as you say it comes and goes. The loose ground is a sound idea (no pun intended
) as that could change as you drive. An alternator issue is likely constantly there, but may change frequency with speed.
More symptom info may help us to provide more help. Does it vary with engine speed, temperature, roughness of the road, etc? Does it always happen when you pass that police station on your way to work?
And so on...
I have a 2001 and have never encountered any loud noise from the stereo, so this is not "normal"... or at least not a standard feature of the 2001s.