Yeah, this one's common and usually not a problem. That click is the mechanical brake engaging.
The first time you run the tool after changing the direction of rotation (using the forward/reverse button), you'll hear a single click on start-up. It's normal. It does not indicate a problem or damage.
IF THAT DIDN'T WORK: If the clicking is continuous, rhythmic, or happens during operation (not just at start-up), that's different. Check for a damaged or worn chuck, or debris inside the gear housing. Have it serviced by a qualified repair person.
HEADS UP: For regular maintenance, blow dirt and dust out of all air vents with clean, dry air at least once a week. Always wear ANSI Z87.1 approved eye protection when doing this.