Hum? Rattle? Grinding?

My 2016 prius makes a weird grinding/humming noise. It seems like it mostly happens when I accelerate from a stop, with higher RPMs (though I feel like I have heard it other times as well). It sounds like something scraping a hollow metal item?! Or like a fog horn?!

I intentionally gas and release twice for the recording's sake.



I imagine my issue has been addressed before. I've read through many threads but can't decide if this sound is what a person calls "rattling." Or is this sound a "grinding" like some posts I have read. Perhaps a "humming"?

 

64, 615 miles on it.

Sorry, the car is moving, at probably around 15mph. I pushed the gas twice, not gently, but also not pedal to the medal. A did I quick but steady acceleration, for the entire duration of the humming, then released then pushed again for duration of the humming. It ONLY MAKES THE SOUND IN THE BEGINNING OF ACCELERATING.
If I were to keep accelerating, the sound ends up stopping. It seems like the sound starts off really strong on first few accelerations after starting car... and then it gets less and less each time I need to accelerate.

Let me say that in other words:

After stopping and accelerating multiple times (RPMs have gone up and down as I drive through town), the humming eventually stops all together, as if it's "used to it" now... But say, on that same drive, after accelerating multiple times and hearing no sound because car is used to it.... I then get on freeway. I get going at a steady 70mph for a few miles (not needing to raise RPMs for awhile now but also not having come to a complete stop)... Well, if I then have to break.... when I go to accelerate again, it will make the noise, as if it needs to "get used to it" again.

I feel like I don't make sense. Basically, it seems like the sound happens when RPMs suddenly go up (accelerating from a stop). If RPMs are shooting up multiple times because it's stop-and-go-stop-and-go, it's like the car "gets used to it" and gradually stops making that sound... BUT, the car will need to "get used to it" again, if on the same drive, it has been awhile since RPMs shot up.

To me it sounds like something loose also. Just the feeling I get. I have a catalytic converter shield installed. The CCM Cat Security brand. Originally, I thought maybe it had come a little loose and was vibrating against the frame, or something along those lines. That's what it sounds like! But when I jacked car and checked shield, it seemed fine/snug.

 

I appreciate everyone's help.