The clicking is probably a small nick in one of the gears, even the smallest chip can cause alot of noise or make the mixer click. Is the beater shaft still turning when you have a load in the mixer (bowl is full)
Or is it just clicking?
If the beater stops turning it is going to be caused by stripped gears, however if it is just clicking this would indicate just a nick in the gear. Either way I would recommend opening the mixer and replacing the damaged gear/gears.
Your mixer is considered a 5 Quart Professional, which means it has the same parts as a KitchenAid 6 quart mixer. The first thing you will want to do is take the mixer apart and disassemble the gearing. You will want to clean off the gears with paper towels and wd-40. Carefully inspect each gear once clean and replace damaged gears.
In the professional mixers when a gear strips you need to replace the grease and housing gasket, this is because the gears are all metal and there may be metal shavings in the grease which could cause the gears to strip again.
The most common gear to go bad in the Professional series mixers is the Worm Follower Gear (9706529). Here is a link to that gear on our website: http://www.mendingshed.com/wormfollower.html
Also, here is a link to the service manual for the mixer, this will aid you in taking the mixer apart and putting it back together correctly. http://lib.store.yahoo.net/lib/mendingshed/kitchenaid6qtservicemanual.pdf
We are one of KitchenAid's largest service centers within the US, and all parts we sell are genuine KitchenAid products. All parts are available at our website, www.mendingshed.com