It could also be that worm axle bearings are loose - so before you go and adjust the meshing, as suggested by Nathan, you need to tighten the retaining ring (under black (?) cover, with 2 small holes).
http://www.astro-baby.com/EQ6%20rebu...own%20Home.htm
http://www.astro-baby.com/EQ6%20rebu...%20rebuild.htm