Diagnosis
That's the internal infrared (IR) cut filter. It's a little physical piece of glass that slides in front of the lens when the camera switches to night vision. The buzzing is the small motor or mechanism that moves it, and it's normal to hear it faintly during the dawn/dusk transition period.
How to Fix It
If it's buzzing constantly, 24/7, that's not right. The first step is to force a reboot to recalibrate the mechanism. Unplug the camera from power for 30 seconds, then plug it back in. Listen after it fully boots. The noise should only happen briefly at night when the red IR LEDs come on, and briefly in the morning when they turn off.
If That Didn't Work
If the buzzing is truly non-stop, even in broad daylight, the mechanism is likely stuck or defective. Go into the app's camera settings and manually toggle Night Vision off and on a few times. Sometimes that can jog it loose. If the buzzing persists, the internal hardware needs service.
Heads Up
Don't try to open the unit or spray anything into it. You'll void the warranty and likely cause water damage. If a reboot and manual toggle don't fix a constant buzz, you'll need to contact SimpliSafe support—it's a hardware fault.