Clunking sound in head of KA 600 Professional Mixer

I've got a 6 month old Kitchen Aid 600 Professional Mixer. Purchased it on eBay, brand new, factory sealed. Was assured at the time of purchase warranty was good. KA will not honor warranty because seller was not an "authorized dealer".

I digress--When mixing, the mixer lets out a loud CLUNK when the beater passes the same spot each time... whirrr...CLUNK...whirrr...CLUNK. I'd appreciate any input as we'll be fixing this ourselves, I'm certainly not planning to pay KA's price to repair.