I had muted the individual clips.
Okay, and now I've definitely confirmed the reason for this noise for myself: my daughter has another old S3GalaxyMini - so yesterday I made 2 identical video recordings with both the Mini and my GalaxyS7: in fact, the transitions on the S3's movies are with noise, the transitions on the S7's movies are not.
Now may understand that technically who wants to, but I certainly don't. Maybe one of you has an idea.
I'm going to add the "Still image at start/end of transition " option to the old recordings now and everything is fine with new recordings.
Have a nice weekend, everybody, and thanks 🙂