Listen to my engine in the "Buzzard Lagoon Road" video. When cold or when running above an idle it sounds pretty good, but when hot and idling it makes an awful ticking sound. I have adjusted the valves many times but it doesn't do any good. I have no idea what is causing the ticking sound.
A neighbor has a much newer and low mileage TW and we compared the sounds the two engines made. Mine seemed a little more pronounced than his but basically they sounded the same. I even had a stethoscope and listened to his and my engines. They seemed nearly identical. That is when I stopped worrying about it.
Could yours have made the noise before you took it in for service but now you are intentionally and intently listening for any strange noises so you hear every little noise it makes?
As long as it is warranty work take it back and see what they say. Mechanics make mistakes too. We all do.