The clicking sound is normal for microbolometer-based cameras.  It is a result of the camera's non-uniformity correction, or NUC.

Non-uniformity correction (NUC) adjusts for minor detector drift that occurs as the scene and environment change.  Upon initial startup the camera will NUC frequently.  The image may freeze momentarily and you may hear a "click" sound.  As the camera warms up and reaches a stable operating temperature the NUC will become less frequent.