When did this problem start? Have you recently added or changed any hardware?
I occasionally see this problem, including with this machine with no apparent explanation. And a common fix is a simple "cold" reboot. Have you tried that? That is, shutdown and flip the PSU's master power switch - if it has one - to Off ("0"), or unplug the computer (and any self-powered connected devices) from the wall. Wait about 15 - 30 seconds, then power up and see what happens.
If still having this issue, I recommend you run with just your keyboard and mouse connected for awhile to see if you still hear the alert. This could easily be your keyboard or mouse acting up so eliminate them from the equation first, before connecting any other device.
Then, if no alert, add another device one at a time. If you still hear it with just the mouse or keyboard connected, try different USB ports. If still a problem, swap out the keyboard and mouse, one at time. If they are wireless, make sure the batteries are good. Weak batteries can result in a weaker RF signal which alone can result in disconnects. Weaker RF signals can also more easily be overridden ("walked on") by stronger signals on the same frequency, i.e. by interference.
