"Real" rod knock does not simply go away and it gets worse at higher rpm's and when you hit the accelerator. It will have a sound similar to a whip crack. In early stages, an engine with rod knock will not make the characteristic "rod knock sound" at idle.

Piston slap can be identified because it is "duller" sounding (not a whip crack sound) and will usually go away after engine warmup. Piston slap will occur at idle.

"lifter tick" is a rhythmic clicking sound and will occur at idle as well as under load. A car with hydraulic lifters (like ours) that has been sitting around for a while without having been run may exhibit lifter tick. Hydraulic lifter tick should go away after a few minutes, otherwise this may be an indication of worse engine problems (low oil pressure, low oil, etc)

There are other noises, but they must be identified by recording the sound of the engine and posting the sound on the internet. Otherwise it's like guessing stocks - most of the time you're wrong