My first step would be to get my head in there with the motor off and check very carefully around the fan, that’s still what it sounds like to me. I looked under the hood of our 4R today and with a good light and some acrobatics, you should be able to look and then run your hand around that shroud. I’d look in front of and behind the fan. If you don’t see or feel anything obvious, and you have the tools and know how, pop the belt off and run it. It’ll be fine for a minute or so, no worries and you’ll narrow down what it might be pretty quickly. It “could” be a loose timing chain or maybe the guide for the TC is faulty. But you drove it 600 miles with that sound? No misfires? Wasn’t running crappy? I’d think you would’ve had some of those issues if the chain was the culprit.