Why is there static noise when playing media at zero volume on iPhone?
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!