So piston slap is a known issue with the LH0 3.1 mpfi as well? I can't say I'm particularly surprised. I know the 3100 SFI's are known to have the issue, along with it's cousin the 3400. Honestly I'm not too worried about the noise, as it's completely gone after driving the car for a while to let everything get up to temperature. Driving hard also alleviates the sound quickly. The sudden increase in oil pressure incurred by the engine upon flooring it and allowing it to rev to 5500 must either forcefully pump up my lifters or the temperature increase allows the pistons to expand and fully fill the bore. It wouldn't surprise me even slightly if I have a lifter that is sludgy. Either one sounds plausible.
It also doesn't help that my thermostat seems to now be stuck open. The temp gauge is able to move itself off its peg fairly quickly, but the temp will not manage to make its way higher than a quarter of the way up while driving, unless I'm cruising at around ~60mph (2k rpm) for a long period of time, in which case it'll traverse to about one mark above a quarter of the way up and stay there. The engine will also maintain operating temperature (195-200) if I allow it to idle long enough, and at that point the engine is as quiet as a Cadillac. Anyways, I digress. Because of this, the engine sometimes gets itself stuck in this "in between" state where the tapping is greatly reduced, but still very sightly audible. This only happens if I drive at low speeds and use the throttle very judiciously.
So yeah. The sound is definitely one that only occurs when the engine is cold, and seems to be related to the oil pressure conditions to a point as well.