More often than not a brief rattle at startup is what's called piston slap. It's the piston skirts literally rattling against the cylinder walls. Can't say regarding this model but some motors are known to do it almost from new, others won't do it until the piston to wall clearance is grown quite a bit from new. Loose rod bearings and or wrist pin bearings can cause this too although rod bearing issues not only usually show up when hot at certain RPM's, they tend to also go down hill very quickly.

Piston slap can take forever before it's a concern. Wrist pin rattles nearly as long and rod bearings, well maybe half a season. You'd have to diagnose better if it's even a rattle or a knock (IE too much timing at first).. Piston slap is usually higher pitch and quiet but sharp sounding. Wrist pin much the same but more muted and rod bearing usually a deeper more muffled noise..

Try youtube for piston slap and see if it sounds the same..

Also, has the valve train ever been adjusted? It's due around now if not sooner.. Too much gap on the lifters will cause a startup rattle but usually just gets really quiet once oil is flowing, doesn't normally go totally away. The good news if it's valve clearance is it's adjustable and doesn't need rebuilding etc.

''''''''If it is there only for a very short time it sounds like timing chain or big end knock before the pump has produced oil pressure.