You have surely noticed that when you adjust the volume on your Mac using the keys for Volume Up and Volume Down will produce a bleep or pop (hard to describe) notification sound. If you have your Mac plugged into an external sound system this can become very annoying, mostly because it’s so loud! In some instances, such as using your Mac over a public address system, there’s no way you want that popping sound when adjusting the volume.

It’s an easy fix.

So to System Preferences > Sounds > Sound Effects 

Untick “Play feedback when volume is changed”