There is a clicking sound up in our attic when the upstairs AC is running (not the downstairs, just upstairs) - where the lines run up from the outside AC units. You can also hear it in the walls of our boys room which is where the refrigeration lines run up from the outside AC units. The sound isn't ALWAYS going, but usually in the afternoons and evenings. Any idea if this sound is normal or possibly an issue? If it is an issue, ideas on what needs to be fixed? All help is appreciated!

Usually it is a expansion/contraction noise. You may also notice a rushing sound like water in a copper line. Both of these noises are normal under some conditions.If you unit has not been serviced for awhile you may consider having it done. Many times a dirty condenser/evaporator will use more energy so having the unit serviced might be a good idea from that point.