Brand new KitchenAid Professional 6500 makes clicking/ticking noise when under load
I suspect that one of two things is happening:

The gears aren't aligned correctly, and they're far enough apart that the teeth are just barely touching, so under load, they start slip rather than mesh up correctly.

The sacrificial gear is failing.  Basically, there's a gear that's designed to break rather than have forces get transmitted back to the motor and cause damage that can't easily be fixed.


The first one is obviously a problem.  I don't really see this happening unless it was dropped at some point.  If you saw damage to the box, or the packaging inside the box, it's possible this is the case.
The second one is specifically designed to happen, although I wouldn't expect it to fail this quickly.  It's also designed to be repaired relatively easily.  I would recommend contacting the manufacturer, especially if you can easily reproduce the sound.
Note : I know that when repairing a friend's Kitchen Aid, I got a pair of special pliers to remove some retaining rings ... I can't remember if the sacrificial gear could be removed without it ... but there's also been a whole lot of variants of the Kitchen Aid mixers through the years that look nearly identical, but are different internally.