Why is there static noise when playing media at zero volume on iPhone? I have iPhone 6 on iOS 10.3.
Recently, while watching a video (not stored on my phone, but on YouTube on Firefox browser) with volume set to zero, I put the bottom speaker on the iPhone to my ear (because in the silence of the room, I heard something), and I heard static, despite volume being zero! I closed the tab, which caused the static noise to stop, then opened the Music app and played a song on zero volume, and when I put the bottom speaker next to my ear, the static noise had come back!

Is this normal? Why is there static noise when media is playing at zero volume instead of no noise?

My Bluetooth is turned off, and the Alerts switch (whatever it's called) is turned off (I.e. You can see the orange line next to the volume buttons).
Hmmm...I did the same thing and I noticed the static too! So, I contacted Apple Support via Online Chat with the exact same wording you provided as above and a helpful representative provided the following answer:

"Now light static is going to be normal on your device even if it is fully down because there’s audio playing. You don’t have anything to worry about."

I replied:

"Ok, because I was worried that despite being on zero volume that the audio would suddenly start playing out loud! It's just that I wasn't aware of this before"

To which Toya, the representative, replied:

"Yes I definitely understand. That’s never fun, but you’re good. Now if it ever becomes loud static or screechy you have a problem."

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So there is no issue here!