While cleaning my bike yesterday I noticed that when the rear wheel turned, the bearings sound a bit noisy*. Does this mean they're on the way out? They're sealed right, so I can't lubricate can I? What would need to be replaced if they are finished? the hub?

The bike is a Trek 4300, a little over a year old (a couple thousand km on it maybe).



*I don't recall hearing this noise before, thus my concern. It could just be the first time I've noticed it though..?