That knocker is the software alert telling you about the switch and pop bumper to check. You can disable the knocker on these software warnings in the Adjustment menu.

The game remembers when the last time the switches were triggered. If too long goes by, then it assumes it's not registering because it's broken, so it's telling you to check it.

I have a pop bumper warning on mine which causes this on startup. My pop bumper works just fine, so I'm not sure what's causing my warning. So, because the knocker is loud as hell, I just turn off the knocker on software warning.