If its a Connecting Rod knocking ... You can Pull the Spark Plug Wire off the Plug for #-1 and #-2 and see if the Knocking diminishes when there's no Sparl. If the knock goes away when the Plug Wire is removed ... that's a good sign that the Big End Bearing is Spun or Bad. The Bearing is "Spun" when the half-shell slips inside the Connecting Rod Big-end and the Big-end knock the Crankshaft Journal.

 

i wouldnt worry about it to much.... just be sure to let the motor idle before you reve it up and try to drive on it.....
my truck taps for about 30seconds on cold startup in this freezing cold temps! after it warms up a bit it goes away and i drive it like normal.

nothing really likes the cold temp!!!!

 

I tend to agree with V8DIET. Unlike piston slap, rod knock does not seem to go away after the engine warms up. In addition, engines apparently fail quickly after rod knock starts, whereas piston slap can persist for some time.

This site details a method claiming to be able to diagnose piston slap with high confidence:

http://wiki.answers.com/Q/How_would_...ts_piston_slap