phone randomly makes siri beep

I don't know what I'm doing to trigger it, so I can't purposely trigger it. But a couple times a day, my phone will randomly make the beep beep sound that siri makes when you hold down the home button. I've usually just put it down or locked the screen, but it isn't because I'm pressing the home button. I've tried turning the volume of siri down, but it still does it. Anyone know if it's a setting I've selected or just a bug.

I have "hey siri" turned off, but this shouldn't matter because this doesn't happen when my phone is plugged in.