The AU is just Minelab's way of telling you your 600 doesn't have manual noise cancel....like the 800 does.....it's ONLY auto.
You can actually start the AU process two ways on the 600:
If you go to noise cancel, the 'normal' way to start the Auto process (on 600/800) is by pressing the Accept/Reject button. (the check mark / plus sign labeled button)
Because the 600 won't allow you to manually change the NC number, Auto Noise Cancel will also be initiated on the EQUINOX 600 if you try pressing the Minus (–) or Plus (+) buttons.
And Fastdraw is correct. The 600 has always worked this way.
However, as noted by others, it is (most annoyingly) not in the manual!
(I had to call ML directly to find this out, when I first got my 600...)
It's working as it's supposed to....relax, and have fun,
mike