What would be some of causes for a knock in the motor?

Running engines make a lot of noises, but when someone tells me that someone hears a "knock", I want to hear it myself. A knock that can be heard at idle is generally an indication of a serious mechanical failure...engine main and/or rod bearings and piston clearances come to mind. A knock that sounds like pebbles rattling in a tin can only under acceleration is a different problem, and is called "spark knock". This is when the fuel mixture in the engine lights off before it's supposed to and is caused by fuel octane rating too low for the engine, advanced ignition timing or excessive carbon buildup in the combustion chamber. Hope this helps and for more tips, see my bio page...