I'm with CO. In the first vid, you can clear hear the rattle that I think you are describing. I can't imagine a scenario where that'd be detonation, especially at low rpm and no load.
Since it's periodic, I'd suspect something running near engine rpm, but either faster or slower, so you're hearing a harmonic which syncs and "unsyncs" with engine rpm sounds (water pump, fan, alt).
All of that said, the sound is peculiar in that it is particularly metallic, and is not suggestive of those components as they don't seem to have the "brass" to make those sounds. What a lot of help, right?
Look beyond the motor!