Static noise issue when video recording

I feel like I would like to create a thread instead of doing this in other thread, so here is what happened to my device:

I found this hissing sound issue on both my iphone6p and iphone7p, but the difference are, iphone7p's hissing sound is louder. However, this is not the biggest issue. So I went ahead for a comparsion between iphone6p's video recording and iphone7p.

By shooting the same stuff at the same time, you could hear a very loud static noise from iphone7p's video clip by comparing to iphone 6p's. So I contacted Apple and they instructed me to go to Apple store for checking, without surprise, they said the unit is all right.

After I went back home, I did another test, same issue. However, if you plug in headphones and then record using iphone7p, you get almost identical sound quality to iphone6p(basically no static noise). In addition, if you use the front camera, the sound of the video is almost the same. Moreover, if you use voice memo, then there's no static noise in iphone7p's recording.

Conclusion is, I really don't think this is normal, and after further chatting with Apple, they promised to do an replacement for my device.

For me, hissing on the back of the iPhone isn't that a big deal, but it will be one if somehow the sound quality of the video get affected...

EDIT: if needed I could upload both footage from my iphone6p and iphone7p for comparison.(poor internet...)