I hear a noise from the bottom speaker of my iphone when audio is playing with sound off

I have an iPhone 6. I use it only for 7 month. I noticed one strange thing. When I play a song or watch a video (anything with sound) with the volume turned all the way down in control center, I can hear a white noise, or a hissing sound. Some sound. Like hissing from the bottom speaker. It only can be hear with volume off and in a very quite room or when I hold the phone near my ear. I noticed the same thing on my iPhone 5. Although I have tried other iPhone 6 and 7 and they don’t make this sound. Is it ok? Can you try it on your phone? What can it be?

iPhone 6, iOS 11.1.2
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