Ever since I bought a Sky-watcher EQ-6 mount back in September 2008, I was disappointed with the rattling noise when starting and stopping slews on speed 6 and above. This occurred on both the RA and DEC axis, and it seemed to be a common problem for new EQ-6 and Orion Atlas owners. Although the mount performed well, the noise was very annoying.
Here is a video of the noise on the DEC axis.
The RA noise was very similar, but perhaps not quite as bad. Fortunately it can be fixed quite easily.
Note - the procedure described below only applies to older EQ6 models. Newer models have a different design and the adjustment is not as easy to make.
When you have the stepper motor out, check the play of the transfer gear. If it's too loose, then this is most likely the cause of the rattling noise - it will cause the motor to resonate at certain points during the ramp up and ramp down. The following video shows how loose the tranfer gears were in my mount.
Before tightening the transfer gear, loosen the retaining grub screw with a hex key. This is important - if you don't loosen the grub screw first you will strip the thread of the hex nut.
Next, tighten the hexagonal nut a little with a socket wrench just enough to remove the excessive play. Don't over do it - you don't want to bind the stepper motor.
There are web sites around to help with the reassembly - the most important point is to get the worm adjustment just right - you should still be able to move the gear with your finger, but with little or no play in the axis. It's worth taking your time to get it spot on.
Here is the end result - no more annoying rattle noise!