A short skirted forged piston with excessive clearance in the bore will cause " piston slap" which is basically the piston rocking in the bore. When the piston warms up it swells in the bore taking up clearance and the condition may lessen or pretty much completely go away.
If it doesn't go completely away, it can and will cause excessive and premature wear in the bore. The only fix is either new pistons or perhaps a coating.
Both Subaru's( which the car dealership I own specializes in) and GM 6.0 liter v8 engines are notorious for this malady. Both utilize a short skirt relatively high compression design
I stay away from any such cars that exhibit this trait if it doesn't go completely away almost immediately after startup.
In a race motor where long term high mile use isn't the norm, I would be less concerned.... But the piston rocking in the bore to that degree definately isn't a good thing
It's definately because the piston isn't sized correctly if it's a new motor....
For whatever reason, Marvel mystery oil seems to help this condition, at least to the ear.
Why I don't know... Lol