In this video I have some pretty good audio of what "piston slap" sounds like. I also give a brief and superficial explanation of what "piston slap" is.

Please note: In order to determine 100% what is causing a noise in your engine it must be properly diagnosed. The use of equipment such as a "mechanics microphone" or a stethoscope should be used to get a better idea of what area of the engine is the noisiest. Engine tear down is needed to pinpoint the point of failure. So, that being said just because you have a noise similar to mine does not necessarily mean that you have "piston slap". The video is just an example of what it may sound like.