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Is the noise you hear kind of a ratcheting sound? If so, it is because the torque required to move the mount exceeds the torque being applied by the motor.
The stepper motors will make this noise and stop turning the worm if the mount is out of balance, or if the gear mesh is tight in a certain location where the worm contacts the wheel or ring gear. If you suspect that your mount is out of balance, then you should reference the PMC-Eight User's Manual to balance your mount. If your mount is well balanced, then your mount's gear mesh probably needs to be adjusted. Another symptom of this is that your mount will stop tracking or slewing.
For further information If you are having problems with your EXOS 2 PCM-Eight mount, then contact Customer Support at +1 (866) 252-3811 ext. 2. If you are not comfortable adjusting the mount yourself, you will need to send it back to Explore Scientific for adjustment.
If you are having problems with your Losmandy G11, there are instructions on the Losmandy website on how to adjust the gear mesh for the mount here:
http://losmandy.com/wormbacklashadj.html
If you want to make this adjustment yourself, for either the G11, EXOS 2, or iEXOS 100 mounts, It is suggested that you disconnect the motor coupler (G11), or remove the belt (EXOS 2, iEXOS 100) prior to adjusting the worm and then after it looks and feels good when rotating the worm by hand, reconnect the motor.
You should seek out additional advice on the PMC-Eight Groups.io Forum, the members there can give you additional advice on how to do this. You can also search for Losmandy G11 forums for additional information and advice for your G11 mount.
Again, If you are not sure about doing this, or want to have Explore Scientific adjust your mount, then you should call Customer Service at +1 (866) 252-3811 ext. 2 or email service@explorescientific.com