I, too, love the Soundcore!
I have what sounds like the same problem and I've just contacted Support to find out what to do. I listen to Classical music on my speaker, and it seems to happen when the music is quiet. I wonder if the speaker "blips out" when it can't detect music, and blips back in when it can?
I'm using a Lumia 950, a Windows Tablet and a Windows Laptop - same problem with all three, and no problem with my other Bluetooth speaker setups around the house and car.
I will post further when I hear from the Support people.