engine has a steady knock

As driving around 25 mph, heard sound as if something fell down below the car. A knocking sound immediately began after that. Any ideas? On further investigation it seems like the sound is coming from the general area of the belts but not created by the belts. As the car idles it is a steady knock but if you rev the engine or put it in gear the sound speeds up or slows down depending on the engine RPM and auto-clutch.