what could be making a knocking sound in the engine?

It isn't easy to diagnose a noise through the net. Is it a loud knocking sound from the lower part of the engine? Front or rear? Is it a very rhythmical or a double knock? Loose connecting rod's pin would make a "double knock", 2 knocks at Top Dead Center (TDC). An expert tech should be able to tell. Stethoscope would be helpful also. Zee