Presuming you have lots of cycles on the engine in the last couple months and the noise has not changed (worsened) it's likely not an issue. A few things that come to mind however is valve train, piston/cylinder clearance, and tensioners. These items do have differences when cold versus warm which could make various sounds.
Sometimes exhaust systems can have loose or rubbing components which rattle a little when cold or warm due to expansion. Think - heat exchanger boxes on the headers.
Tough to be of much help without being there to listen with a stethoscope, but if the noise is minor, consistent, and dissipates, then it's probably a non issue.