Under some conditions, there can be repeated, clearly audible “clicks” in sound on at least Debian 10 and 11 (Buster and Bullseye) GNU/Linux, accompanied by momentary audio output device switches. Web searches indicate that other distributions (at least Debian derivatives) are affected as well; I have been able to locate cases where Ubuntu and Mint users have both been affected by this type of issue.
I haven’t dug very deeply into exactly why this happens, but it seems to be somehow related to ALSA port availability changes; which is kind of odd when it happens without any changes in what hardware is available.
The fix, however, is actually quite simple. Open /etc/pulse/default.pa in an editor running as root:
$ sudo nano /etc/pulse/default.pa
Locate the line
load-module module-switch-on-port-available
Prepend a # to comment it out:
#load-module module-switch-on-port-available
Save the file and exit the editor (in nano, by pressing Ctrl+O, confirm saving, then Ctrl+X to exit), then under the user account suffering from this problem, stop the running PulseAudio daemon.
$ pulseaudio --kill
A new PulseAudio instance should start as soon as it is needed, reading the new configuration as it does so.
This should resolve the issue.