Worm gear backlash/issue.
I had the same thing on me HEQ5 on both RA/dec after a few months of use. But not 2cm worth, enough to cause issues though.
Tighten the worm gear (a little bit) and it will go away. Just don't over tighten it or you will get stricture or, even worse, binding (your motors will make a horrible grinding sound).
I know this is only for the HEQ5 but i cant find one for the EQ6, but i suspect it is similar;
http://www.sky-tron.com/heq5-rebuild/heq5-we1.htm
The screws are probably in different positions but you should be able to tell which is which.
Follow the advice listed on the page;
- Run RA/dec through a full cycle to make sure there is no binding. If there is binding, loosen what you have done (a little, and i mean a very small amount).
- Make sure you can turn the gears with your fingers. If you can barely turn them or require a lot of force to turn them then it is just a little too tight, losen it a bit.
Also, remember that due to gear designs there will always be a little bit of backlash so don't try and get it super tight or binding will occur.
Good luck!