How to identify/eliminate system sound generated by SystemUI that defeats DND

I have a recurring notification chime (whatever my default system notification sound is set to) just about every 3 minutes, even if DND is set (defeats DND even with no app notifications allowed). There is not an actual notification that will show up in the notification history when these sounds happen. The device is a US Galaxy S21 Ultra (SM-G998U), not rooted running stock AT&T image: OneUI v6.0, Android 14, Kernel 5.4.233-qgki-27920445-abG998USQU9FWKC, build UP1A.231005.007.G998USQU9FWKC I ran: adb shell dumpsys audio > %filename% and found the following events responsible for the sounds (systemui every time):

01-11 23:15:36:546 requestAudioFocus() from uid/pid 10064/2832 AA=USAGE_NOTIFICATION/CONTENT_TYPE_SONIFICATION clientId=android.media.AudioManager@a5ae8cc callingPack=com.android.systemui req=3 flags=0x0 sdk=34
01-11 23:15:38:452 abandonAudioFocus() from uid/pid 10064/2832 clientId=android.media.AudioManager@a5ae8cc
01-11 23:17:35:853 requestAudioFocus() from uid/pid 10064/2832 AA=USAGE_NOTIFICATION/CONTENT_TYPE_SONIFICATION clientId=android.media.AudioManager@41872a9 callingPack=com.android.systemui req=3 flags=0x0 sdk=34
01-11 23:17:37:657 abandonAudioFocus() from uid/pid 10064/2832 clientId=android.media.AudioManager@41872a9
01-11 23:18:36:586 requestAudioFocus() from uid/pid 10064/2832 AA=USAGE_NOTIFICATION/CONTENT_TYPE_SONIFICATION clientId=android.media.AudioManager@94ac65a callingPack=com.android.systemui req=3 flags=0x0 sdk=34
01-11 23:18:38:594 abandonAudioFocus() from uid/pid 10064/2832 clientId=android.media.AudioManager@94ac65a
01-11 23:21:36:616 requestAudioFocus() from uid/pid 10064/2832 AA=USAGE_NOTIFICATION/CONTENT_TYPE_SONIFICATION clientId=android.media.AudioManager@9eb1d5f callingPack=com.android.systemui req=3 flags=0x0 sdk=34
01-11 23:21:38:425 abandonAudioFocus() from uid/pid 10064/2832 clientId=android.media.AudioManager@9eb1d5f
01-11 23:24:36:683 requestAudioFocus() from uid/pid 10064/2832 AA=USAGE_NOTIFICATION/CONTENT_TYPE_SONIFICATION clientId=android.media.AudioManager@e82a8fa callingPack=com.android.systemui req=3 flags=0x0 sdk=34
01-11 23:24:38:512 abandonAudioFocus() from uid/pid 10064/2832 clientId=android.media.AudioManager@e82a8fa
01-11 23:27:36:839 requestAudioFocus() from uid/pid 10064/2832 AA=USAGE_NOTIFICATION/CONTENT_TYPE_SONIFICATION clientId=android.media.AudioManager@4df9790 callingPack=com.android.systemui req=3 flags=0x0 sdk=34
01-11 23:27:38:673 abandonAudioFocus() from uid/pid 10064/2832 clientId=android.media.AudioManager@4df9790
01-11 23:30:36:790 requestAudioFocus() from uid/pid 10064/2832 AA=USAGE_NOTIFICATION/CONTENT_TYPE_SONIFICATION clientId=android.media.AudioManager@256f4a4 callingPack=com.android.systemui req=3 flags=0x0 sdk=34
01-11 23:30:38:579 abandonAudioFocus() from uid/pid 10064/2832 clientId=android.media.AudioManager@256f4a4
01-11 23:33:37:461 requestAudioFocus() from uid/pid 10064/2832 AA=USAGE_NOTIFICATION/CONTENT_TYPE_SONIFICATION clientId=android.media.AudioManager@fcd7dd callingPack=com.android.systemui req=3 flags=0x0 sdk=34
01-11 23:33:39:329 abandonAudioFocus() from uid/pid 10064/2832 clientId=android.media.AudioManager@fcd7dd
01-11 23:36:37:508 requestAudioFocus() from uid/pid 10064/2832 AA=USAGE_NOTIFICATION/CONTENT_TYPE_SONIFICATION clientId=android.media.AudioManager@d180e23 callingPack=com.android.systemui req=3 flags=0x0 sdk=34
01-11 23:36:39:321 abandonAudioFocus() from uid/pid 10064/2832 clientId=android.media.AudioManager@d180e23

PID 2832 is in fact com.android.systemui - so it seems apparent that the sounds are generated by systemui. How can I get it to stop? You don't appear to be able to block systemui sounds. Any idea what could be causing it or how to troubleshoot further?