Those auto-locking hubs have a very weak plastic "cam" inside them, and when that wears or breaks, the clicking and ratcheting sound you describe is one of the common symptoms of the slipping hub. For further diagnostic info, a little about how they work, and also recommendations for upgrading these to a manual locking hub, try:
http://therangerstation.com/tech_library/HubDiagnosis.html
Tom Dayton
JS Auto Supply