Q: Noise under the driver's seat

I purchased the car about a year ago and it's been having many issues ever since then. The car makes a loud noise which is noticeable when I'm inside of the car. The noise is coming from the area under the driver's seat. As I shift gears, I've noticed that the noise is changing. I took it to a mechanic who first said it was coming from the gear. After switching off the car and removing the belt, the mechanic started the car but there was no noise. Then. I was told it might have something to do with the power steering. I had that replaced with a used part but the noise was soon back. Also, if I let the car sit for a few days, the right side on the front end would drop down. What is causing all this? What should I do?