If you hear a low end knock, chances are you have a bad rod or main bearing.
April, 3, 2013 AT 2:19 AM
Try to determineif the noise is coming from the top of the engine or the bottom (you may have to get under the car or at least listen down by the front tires to determine if it is from the bottom of the engine.
When the noise started, was the engine low on oil? Was it running hot at the time?