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Pete, misalignment could be the cause though I can't say I've experienced this myself. The STL11k and standard robofocus should be fine unless you've got an external filter wheel and other accessories.
Does it move in either direction at all?
Focuser draw tube lock is not down is it?
Have you tried recalibrating it to measure the focus in counts? Becareful here if you run FocusMax it you may need to redo v-curve runs.
The humming noise is natural when idle as its holding its position. If you are having camera 'holding' problems where the motor slips when it moves, increase the duty cycle percentage. 50% or 60% is normally enough.
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