Underbody Shield problems of the 2003 Chevrolet Cavalier

I purchased my car with 2 miles on it in may 2003. After 2 weeks of having it there was a knocking noise that started under the front end. The service dept. Said that it was just a new car sound and it would go away. After several months I took it back to the service dept. And he found that there were many bolts to the front end frame were never tightened or put in from the factory. So they put in bolts and tightened the rest. Hoping that would solve the problem I left. It continued to make a popping type sound and I took it back where this time they heard it but could not find what it was. To this day it sounds as though the front end is going to fall out and the steering has changed. The service dept. Said if they can't find the problem then they can't fix it and to drive it until whatever it is breaks or falls out! I still have the problem and I am terrified to drive it. Also the seat belt light does not work properly,the dash has a loud vibrating noise,the heat and air makes a loud buzzing sound, the gas gauge does not work properly and the car does not accelerate and decelerate properly. I have contacted the manufacturer about the problem and they said they were sorry about the bolts never being put in and that their vehicles are inspected and are safe. Well how is that when they did not even put in bolts that holds the motor in?what else have they forgotten?I feel unsafe in my car and am very angry that this problem persists!.