Does your power button feel woobly at all?
This was my first DIY, because the knob didn't control on/off functions anymore when I got the car. I took radio out, opened the top of it and found out that the knob attached to a small circuit board was not supported anymore.
I used small pieces of balsa wood to make a support for it. I did this soon enough before the wires started to get turned around.
In your case, could the knob (if not attached anymore), have turned and caused a short at one point and resulted in the out-of-control loud noise? I would look inside the stereo, and get the old soldering iron out...