Dell Soundbar scratchy sound after some time on Ubuntu 18.04

Hey
I've installed Ubuntu 18.04
Sound present. Still on my Dell AC511 USB Soundbar it's scratchy from time to time.

How to reproduce:
1. make active Dell AC511 (Digital or Analog doesn't matter) active device for output in Settings=>Sound
2. start playing sound (tried with rhythmbox and youtube inside browser)
3. sound is playing nice for about 30 seconds, after that it becomes scratchy and much more loud. After some time (about 15 seconds) it becomes again normal, after that again scratchy and repeat again. Time is not constant, it changes.
4. if I had active Digital Output selected and sound becomes scratchy switch to analog output helps for about 30 seconds and vise versa

It looks like the problem is in output volume - because when sound becomes loud and scratchy I can decrease volume in sound settings and that helps, but in several settings it becomes silent so I need to increase it again

On Xonar DS with headphones there is no such effect and sound is perfect. Haven't try built-in audio

I've already tried similar steps which described in https://answers.launchpad.net/ubuntu/+source/alsa-driver/+question/670449
to reinstall alsa and pulseaudio without success.

please help
Thank you in advance