I just bought the 'M' model and I can confirm the issue is still there, but mainly because the issue is most severe because of the monitor's USB hub rather than the sound bar itself. I've replied to a slightly older thread on AC511 buzzing white noise. Essentially don't use the sound bar via the Dell monitor's USB port at the back, even though via the side ports was slightly better for my monitor (U2717D) it was still not acceptable in a quiet room with my head not far from the monitor. You need to connect connect directly to the pc/laptop/ps4.

I have a Thinkpad, XP13 and a Macbook Pro connected to my monitor, instead of just switching the single upstream USB cable to the device I use, now I also need to switch the sound bar. It is absolutely disappointing. I wonder if other USB sound devices are this bad at being affected by noisy USB hub.

We have many of these, some several years old, some brand new. The noise floor is poor and noticeable if you are in a quiet room. Same result regardless of usb ports plugged into, different Dell machine types, different buildings. Even with the volume at zero, you still hear it.  I just tested a few today.

I had similar issue and it seems that the soundbar is very sensitive to wireless adapters situated nearby. I do have USB WiFi adapter next to my LCD screen, as soon as I moved it away the noise disappeared. The right part of the speaker seems to be most sensitive. 

Hope that helps.