Your rattle/vibration in the footwell could be a loose heat shield on the main cat or something else on or around the exhaust & y-pipe. If you feel it in the steering read on...
I had a humming noise and slight vibration that would grow in intensity from about 35 to 60 MPH. After replacing one wheel bearing and starting on the second one, I discovered the left side inner CV joint was very worn. This was around 120K miles, so the wear was not unexpected. NAPA has the axles for about $65 and it's quiet as mouse now!
