Tapping the screen produces rattling sound

Nope, no bumper or case of any kind. I tried holding the various buttons (volume, hold/power) in place while tapping away, and the rattle was still there.

However, I just noticed if I hold the phone tight around the sides, the rattle goes away. If I tap while it's laying in the palm of my hands, the rattle comes back.

Just to close up this thread. I went to the Apple store, genius duplicated the issue on my phone and his phone as well ⚠. I opted to get a replacement phone, and no longer hear the screen rattle when tapping on the left side of the screen. So, a replacement fixed the issue for me.