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..?

It'd be quite rare to get a rear wheel to such a state that you can actually hear the bearings. Are you sure it's not the clatter of the pawls in the freehub that you're hearing?

If it really is the bearings,then it's not a good sign. But a clean and a lube can do wonders.

A quick google has your bike down as being equipped with a cup & cone Shimano rear hub, which can be disassembled, cleaned and lubed with a moderate effort.

Well, cones and balls can be replaced, if indeed that's where the trouble is(which isn't all that probable)
Cups can technically be replaced, but aren't available as spares. If yours are scored/pitted you'd need to find a donor hub first.
If you're having someone else do the work it'd probably be cheaper for you to simply buy a replacement wheel.

If it's just contamination, freehub bodies can be disassembled, cleaned, lubed and rebuilt w/o going to heroic measures. They can even to some degree be flushed and relubed w/o disassembly. Most people would probably just replace them though.

"Noise" is too vague to use as a basis for a diagnosis. Is it crunching, squeaking, or ticking?
I suggest you drop the wheel out of the frame and twiddle both the axle and the cassette with your fingers. That'll tell you far better if things are moving freely or having too much play than with the wheel in the frame.
The axle should have virtually zero play back & forth, and should turn easily in your fingers.
Some spots of resistance might be accepted, and certainly don't expect it to keep spinning once you stop twirling it.
Cassettes should also turn freely in one direction, and engage quickly in the other. Some wobble is acceptable.

Thank you very much.

(The noise, btw, is a kind of dry scratching - like something needs to be lubed...)

Sounds like the bearing are a bit gummy. I think hub overhaul -- disassembling hub, cleaning cups, cones, and bearings and regreasing (you grease bearings, not lube, by the way) -- ought to quiet your wheel.

https://www.parktool.com/repair/readhowto.asp?id=105

Although repacking and adjusting the bearings is always a good idea, you might simply have gotten some grit trapped at the base of the cassette/freehub.