Yes it is probably condensation. Ductwork should be insulated, on the inside, or outside to prevent this. Wrap the areas with fiberglass, and tape the ends. If left unattended it may cause a musty smell in the ductwork.
Is the insulation tight on the duct or loose. Somewhere warm air is making contact with cold air. A small amount of moisture is not unusual during an extended period of warm weather, but once the insulation is saturated, it is not insulation any more, it becomes a conductor.