Could the shop have messed something up changing radiator?

If your motor mounts are shot then the engine could shift causing the fan to hit the shroud.

What I don't understand though is that it only does it when the engine is cold. Why would this be?

Fast idle?

Normal cold idle. Not sure I would call it fast but being cold, little faster than usual. I watched it yesterday and I couldn't see the fan hitting shroud. It's weird, at times it's more noticeable when put in reverse which may lead a person to believe transmission issue but it will do it just sitting in park. All only when cold. (starting in the morning after sitting through the night) This started less then 24 hours after having radiator changed. Related or coincidence? I don't know, but odd.

It's very common for weak motor mounts to let the fan hit the shroud. Only when cold sounds odd. I like keeping it simple. It sounds like a fan hitting the shroud, they had to remove the shroud so it should be the shroud. Maybe they didn't line it up right?

After going out again this morning to check it out, it is the fan hitting the shroud. Did some more research and came to the conclusion that it could be one of a few things. Loose shroud, fan clutch, water pump or motor mounts. Crossing my fingers for just a loose shroud as that is probably the easiest and cheapest. Sounds like it could likely be the fan clutch if not loose shroud. Because it only does it on a cold start it sounds like motor mounts might be ruled out.

When the engine is cold it idles faster which could shift the engine enough to make this noise . The thing to do is raise the hood and have someone start it while observing the fan and shroud with a good flashlight.

The shroud just didn't get put back on right. Or the radiator didn't get pushed in all the way. Never seen a fan clutch go bad and do that.