It is not a biological clock (except in the vaguest most possible sense).
It is not an explosive device.
It is not poorly-tuned bamboo wind chimes.
It is not the pad of little cat feet which need their claws clipped.
It is not the moon rattling in a corner of the sky like angry candy.
It could be the carbon dioxide bubbles bouncing off the sides of my soda can, but it is not.
It could be the leaves of the ficus tree delicately giving in to age and gravity and falling to the floor outside my cubicle, but it is not.
It could be the sound of my forehead hitting the desk in disbelief over and over as I read the headlines, but it is not.
It is instead the second hand of my watch taunting me as I try to find something useful and worthwhile with which to occupy this final hour. Maybe I'll go get the windex and clean/dust my desk. That's a suitable Friday afternoon activity...