What app/notification is causing this sound on my MacBook Pro Early 2011 My MacBook Pro running macOS 10.13.6 sporadically makes the following sound (see here for .mp3 format). I'd really like to know where this is coming from. Sometimes it looks like the computer makes the sound when the Ethernet cable and/or the power cable are disconnected, but I haven't been able to reliably link the sound to any of these events as the sound is not played every time they happen. Even just being able to record the sound using my phone was lucky. Also, normally this sound is made a few times over the course of a few minutes and that has been happening once every day or two for the last week.
I have followed the suggestions in the answers here and here. As the sound is played infrequently, some of the solutions haven't yet turned up any results. Every time I restart the mac the first thing I do is start the following command on Terminal: sudo fs_usage -w | grep -i &quot;\.mp3\|\.aif\|\.m4a\|\.wav\|\.caf&quot;. The sound has played multiple times while the command is running but no audio file with that sound is shown in the command line.
I have looked at the Notification Center notifications and, although I have some from around the time this sound was played, none of those programs make this sound. I have also looked at the Notifications preference pane in Settings and no app in that list makes this sound (that I'm aware).
I've also searched the following locations for any .wav, .aif[f], .mp3 and .m4a file:

/Library
/System
/Applications
~/Library

I played all the sounds I could find, but my search has come up empty. Naturally, the sound could be saved in another file format or be in another location...
Sound files are stored in ~Library/Sounds, Library/Sounds or in System/Library/Sounds folder. Look in these 3 folders and see if that particular sound file is there.