Talked to the Apple Support today, he said it is clearly not normal. I made a video in a quiet place so that they can hear the hissing sound. He made a report directly to the engineer department and he will receive an answer in 24 hours, tomorrow I have a meeting again with him. Then talking about next steps. It is important to report that problem to the apple support, that they can find a fix for all. Could be software or hardware related. And in case of hardware, he said, I will get a new replacement straight away.
I have a iPhone 17 pro 256gb. I don’t know anybody who had this issue before, including me.