Problems with humming sound

Humming could be just about anything. Here are a few questions that may narrow it some. Does it make the sound with just the engine running, in park? Does anything you do make the sound louder or change in pitch, like speeding up, braking, turning? Does a change in road surface make a difference like blacktop, concrete, dirt? If you steer left and right while it is making the noise does it change? If you drive next to a wall or building can you hear the noise with the window down? Can you feel any vibration in the seat or steering wheel?

https://www.2carpros.com/articles/car-howling-noise

This is some info and noise for a bad wheel hub bearing:

https://www.youtube.com/watch?v=YvKWGjmPKoc

Is it a tire or bearing issue? Quick tire wear check.

https://www.youtube.com/watch?v=QWP6CSVw_V4

Both can create the same sound which makes it harder to narrow down. They can also make a lot of noise but when checked have no play. Couple ways to tell, drive it and while it makes the sound gently swerve back and forth. The idea there is the weight shifting will make a bad tire or bearing change in sound as the weight shifts. So say the noise gets louder when you swerve left. That usually means the opposite side is the bad one, so loud left means bad on the right, and louder right means bad on the left. Then put a jack under that side and check the tire for uneven wear what you want is nice even wear side to side and all around the tire. If you see tread worn in scalloped patches the noise could be the tire. Then check the bearing for play both in vertical and horizontal directions as the video shows. Another trick is to reach behind the tire and place a bare hand on the spring and then rotate the tire with your other hand. A bad bearing will usually show itself with the spring vibrating lightly in your hand. A tire won't show up that way.

On a good tire it shouldn't although it isn't helping the tire. Now if the tire was at 10 psi when it should be 34 or so you may want to check it close for damage, but if it was say 28-30 with a required pressure of 34-36 then it shouldn't be an issue.

You may want to check with Nissan first. That should be covered under the power-train warranty for 5 years 60,000 miles. No sense doing it yourself if they will do it free.

If they are doing it under warranty I would let them. Sounds can be misleading as to location because it can travel through the body of the vehicle. I've heard bad bearings that I would swear were in the front right, but when I put the chassis ear on them it turned out to be a rear bearing that was actually bad.

Did they say what was bad about the axle? There are multiple things that are possible but on an 18 I would be suspicious that it needs replacement unless it was bent or cracked. Oh if you wanted to look for the new part it is part number 55501-9VB1B looks like it has already been updated a few times, so it might not be a bad idea to contact Nissan corporate and talk to them if the local dealer insists it be replaced but doesn't give you a good reason. From the looks there are only the two bushings in it to go bad.

I would find out what they claim the problem is. The only part that isn't easily replaced is the axle beam itself. Everything else is a bolt on part like the hubs, brakes and suspension. If the issue is something like a bad strut or bad springs then they are replaceable as individual parts. The beam itself is replaceable but unless it's damaged in some way I cannot think of a reason to replace it so I would try to find out what is actually wrong. For instance say they want to replace it because it is bent on one end or in the middle. Do you have any strange tire wear on the tires from that end? That might give a clue to a problem.
I just am not a fan of replacing expensive parts without knowing why.

As far as replacing it, the manual parts are not really hard but the factory data assumes you are replacing the entire rear axle as an assembly with new hubs, brakes and all of that attached. In your case I would try to reuse all of that if possible which is why you need to know what is wrong. Wouldn't help you to replace the axle beam if the problem is really something else.

1. Remove the reservoir cap from the reservoir tank.
2. Remove the rear wheels and tires.
3. Loosen the flare nuts on the brake lines,
4. Remove the rear drum brake assembly.
5. Separate the parking brake cable (RH/LH) from the rear suspension beam and position the rear drum brake back plate (RH/LH) aside with wire. Refer to Exploded View.
6. Separate the brake hose (RH/LH) and the brake tube (RH/LH) from the rear suspension beam. Refer to. Exploded View.
7. Disconnect the harness connector from the wheel sensor (RH/LH). Refer to Exploded View.
8. Position a suitable jack under rear suspension beam.
Place the jack in the center of the suspension beam.
Do not damage the suspension beam with jack.
9. Separate the shock absorber (RH/LH) from the rear suspension beam and remove the coil spring (RH/LH).
Make sure the rear suspension beam is stable when using the jack.
10. Remove the rear suspension beam bolts.
11. Slowly lower jack, remove rear suspension beam from vehicle.

How do you mean different axle? You said the back axle, that is the rear beam suspension for a two wheel drive Kicks which was the only one sold in the US. I don't see any of the Kicks sold elsewhere which have a rear differential either.

That could be the issue, which replacing the beam would correct for a while. It might be possible to bend it back as well, you might want to have it checked for 4 wheel alignment before replacing it though. That would show if it's bent and how much.