This will be of absolutely no help... but "hum" 🎶? Yeah I'm not sure how to fix this... But I finally figured out how to Disable the Bluetooth High Volume Warning with Tasker (again) + Tasker's AutoInput... 🐧
I think chmultimedia could be right as this only seems to happen using spotify correct? As I use my Samsung music app through my vehicle bluetooth system and everything works fine... Did you try playing a track on your phone through your vehicle bluetooth if so do you have the same problem?