Radio/Speakers thumping/static noise
I have a 2000 V6 Mercury Cougar (Automatic) with the standard CD/Radio player and stock speakers with 62k miles on it. I went to China in 2006, came back in 2008 for 2 months and drove my car quite bit, and everything was fine. I went back to China and returned in July of this year. My car hasn't been driven that much during these 4 years, but it was started, battery was replaced, no major issues.
Now randomly my speakers will start to make this static or thumping noise - hard to describe the noise exactly. When it happens I can turn the volume all the way down to 0 and it still makes the noise quite loud. Sometimes I will turn the radio off by pushing the volume knob and turn it back on and it will fix itself for some-time, other times it doesn't work. Once I turn the ignition off and start the car again, the problem will also be fixed.
The noise first started (to my notice) after I got on the high way and got off (high speed). I thought I had blown a tire from the noise, but everything was ok. It seems to be coming from my rear left speaker. I had changed the Bal/Focus to only play through the right set of speakers and it was resolved for like a month, but now it has started even when doing that (rear left side)
I have noticed when I turn my headlights on they will often dim/get brighter/dim/get brighter - along with the internal lights in the dashboard, the radio light, and even the overhead lights.
What could be wrong, what can I do, and how much will it cost? I'm not a car guy and don't know much about them, I'm just really tight on funds right now after getting back to the States.
Furthermore, would ignoring the problem cause prolonged damage somewhere? Would replacing the speakers and/or the head unit do any good either?