Chevrolet,Car makes grinding noise

I've noticed some sight noise in my car lately. It is coming from the bottom of the vehicle, where my feet are. This is an intermittent grinding and thumping noise. There is no noise when I first put it in drive upon starting the car. However, as I drive, the noise starts occurring up to the point when I reach around 45 mph. The noise then disappears again. When I put the car in reverse, it doesn't make any noise. What could make this noise? Are there any parts I need to replace?