"Gnawing sound from V3 Cam

When I turn the sound on one of my cameras I hear a rhythmic “gnawing” or “grinding” sound. When I check the room in which the cam is mounted there are no ambient noises. If I tap on something the cam mic picks that sound up but it echoes for 10-15 seconds before dying away. Does anyone have any idea what could be causing the constant rhythmic gnawing sound?