If it has been a long time since you changed the shocks on your car, do that. It might not fix the problem, but it might help. The shocks that come standard on most cars are only rated for about 20,000 miles. Most will go a lot longer than that, but people tend not to change them often enough. A bad shock abosorber might allow road vibration to transmit into the frame and as a result into your feet. Also, climb under the car and have a look around. If you see a cracked weld in the floor boards, or a brace or frame piece that is missing a bolt, or has a broken weld, that might be the problem. Also look to see that the exhaust pipe brackets are not loose or broken. While there look at the shocks and springs at each wheel. Make sure nothing is loose or broken. If you see oil or oily dirt all over a shock, is has leaked and gone bad, get it replaced. Good luck!