Unless they're brand new gears, I hope you checked the tooth contact pattern before removing them from the original housing they were running in. Once a wear pattern is established, any other adjustment, including setting the backlash to specification will set up a howl that would make a timber wolf jealous. Most service manuals have a section on what to do to establish a proper running fit on new gears which is far too complex to address in a chat room post.
Jerry
