Piston slap sounds more like large ball bearing's clattering in the motor, a sure sign of a tired motor. Pinging happens in engines with big miles from too much carbon accumulating on the pistons increasing the engines compression ratio....it becomes more noticeable when running lower octane fuels when under load.
I have driven Manual trans vehicles for the last 35 years and have never come across this. Started the truck, released clutch and it doesn't engage at...