GM-GMC: 1998 sierra 5.7 vortec noise in oil pan, gasket leaks, piston slap

Question
When I fisrt start my 1998 gmc sierra 4x4 5.7 vortec, there is a fairly loud rattling from the oil pan area.
after 5 to 10 second the sound goes away.
my friend mentioned that there may be antifreeze in the oil, (intake gasket leaks were common on some of those engines) there is good pressure on the oil gauge and there is 230000km on the truck.

Answer
If there was coolant in the oil it would be visible on the dipstick, the fact that the noise goes away indicates to me that it is something like piston slap, which, once the piston heats up and expands goes away. That too is not uncommon on some of those engines. Hope that helps.