yeah ive thought of that aswell
hey datsun kid, just like mike said, try this... its a pretty easy thing to try...
First thing in the morning, start the engine up and run it for 15 seconds while you listen carefully and memorize the sound and it's intensity. Shut it down quickly, pull the spark plugs and put two squirts of motor oil into each cylinder, unless you know its #4. Reinstall the plugs, fire the engine up again and listen. If you have piston slap the noise will have been greatly reduced or even eliminated…..for 15 or 20 seconds that is, and then your nightmare noise will come back.
if not, im sure a few other people might have some ideas... but my mits 3000gt just had the same issue and it wasnt hard to crack open the bottom end and see i had spun the #6 bearing... it was $9 fix (i spent $54 total just to make sure all the bearings were new), since we had all the tools in our units motor pool to turn the crank and rework the rods/caps