I did some troubleshooting and found that the sound persists in my test user account (a different sound, though), but disappears with a safe boot, so it looks like it's source is narrowed down to either a launch agent or daemon. (Does that make sense?)
Makes all kinds of sense to me and since LaunchAgents technically are user oriented and owned that would leave LaunchDaemons and kernel extensions (*.kext) as prime suspects. (However I am not convinced that all developers adhere to the generally accepted division between LaunchAgents and LaunchDaemons 🤬)
If the problem is a third party LaunchDaemon or kernel extension it may not (probably won't?) be fixed in MacOS 10.15.6 or Big Sur.
