Various radio services have developed their own ways of dealing with the noise; the business band radio world, the police radios, VHF Marine radios and a ton of others use FM. FM is a constant carrier mode; whenever someone is transmitting, the radio puts out the same power. When the received signal is stronger than anything else coming in the antenna, FM has a capture effect that suppresses the interference, and the circuits aren't as effected by amplitude noise, so that noise is less of a problem. That allows the receiver to use a squelch system that turns on the audio when the carrier appears and turns it off when the carrier isn’t there. That has led to the ability to squelch the audio of other people on the channel whom you might not want to listen do, but open the squelch for people you do want to talk to, via Continuous Tone Coded Squelch Systems (CTCSS).
If you listen to the AM broadcast band (yes, it’s still there) or tune the shortwave spectrum, you hear broadband noise. In general, you’re hearing “white” noise, called that because it contains an unshaped band of noise frequencies all at the same amplitude. It’s called white noise is by analogy to white light, which is all the colors (which are different frequencies) combined. White noise sounds like a hiss and if you tune through unused parts of the spectrum with a receiver with no squelch or audio muting, it’s all you hear. Many people call this static, but the most common term hams use is QRN; one of the many Q-signals that hams have developed for sending in Morse code.
In Communications Theory, it’s referred to as AWGN, Additive White Gaussian Noise, in case you come across that term.
If plain white noise was all you heard, you’d really have few problems. White noise has been studied to death and it’s a rare (very bare bones) receiver that doesn’t have a noise blanker and often a noise reducer (NB and NR), although their effectiveness can vary. The real world is full of noise sources of all sorts. Ignition systems in passing cars produce a ticking or clicking sound, impulse noise, when every spark plug fires. Switching power supplies, which are built into every LED bulb and many “wall wart” chargers for phones and other things, produce a broad spectrum of single tone-like spikes through the HF spectrum (some work at higher frequencies). Distant lightning strikes cause impulse noise. Electric fences make noise. Many power tools and household appliances make noise; welders make especially bad noise. Grow lights, fluorescent lights, appliances like air conditioners or refrigerators, solar panel systems (the DC-AC inverters), aquarium heaters, electric light dimmers and more.
I’ve noticed in my (relatively limited) time on the 80 meter band that I hear thunderstorms when cold fronts are a couple of days away as well as when they’re almost here. From here, when there’s a cold front over the southeast US, from say Louisiana stretching over Alabama and northern Georgia to North and South Carolina, I hear those storms. Then I don’t hear them as the front gets closer, but then hear them again when the front is close. 80 and 40 meters are worse than higher bands for storm noise. The ability to hear storms both near and far is a bug, not a feature and makes those bands much less usable during the summer months.
This is just barely scratching the surface, and it might give the mistaken impression that these sources sound like each other. That’s only in the broadest sense.
One of the advantages of the modern Software Defined Radios, or radios enhanced by the addition of some DSP and software features, is that you can see the disturbances on a graphical display. In the late ‘90s through the ‘00s, the amateur radio manufacturers started to add spectrum displays that allowed you to see the band you were operating on. This led to people noticing odd noise patterns on screen that corresponded to noises they were hearing. Sites like this one tried to create catalogs of what was seen and what it was coming from. The ARRL has a very good page of pages on RF interference, noise and one similar to that first link but more encyclopedic. Small SDRs like the RTL-SDR can be used as spectrum analyzer to help find noise issues. This pair of spectrum plots from the NK7Z site shows an 80 meter noise problem when present (left) and absent. He's not hearing that low signal on the right while that noise is there, especially if it moves in frequency (as many types of noise do).
Coping with noise
Noise blankers are generally circuits that remove noise spikes like ignition noise rather than more broadband noise like white noise. Sources like ignition noise, distant lightning strikes, electric fences, and a ton of other things.
Because white noise is random and full spectrum, it can be reduced by simply averaging the audio. Over the time periods of most modulation, the noise changes many times while the audio doesn’t. You might see this referred to as correlation filtering in Digital Signal Processing; the algorithm keeps correlated samples like voice and throws out uncorrelated sounds like noise.
Tracking down the source of noise problems can be rough, but there are many example stories you can find online. If you have a rotatable antenna, you can sometimes tell which direction the noise is coming from, but if it’s not coming from something like a utility box you can report, that’s unlikely to do you much good. If you point the antenna toward a neighbor and realize you’re hearing them welding or working on a street rod, you’ll probably have to work around that, using the radio when they’re not working. Sorry, but I know of no way to get rid of all of that noise.
In the best case, you can find it’s coming from something in your house, or that you have control over. Noise from most electrical things can be dealt with by some combination of putting the wiring through ferrite cores or wrapping the wiring around a larger ferrite toroid and possibly capacitors to ground. In other words, filtering. Sometimes it requires a few filters to suppress the noise sufficiently.
If you have the ability to reposition your antenna, that’s always a good thing to try. Say you stretch out a random wire in your yard and you regularly hear a lot of noise, see if you can move that end so that’s not pointing in the same direction. Pointing it 90 degrees away should make a big difference if that’s where the noise comes from. A particularly bad place for an antenna is in your attic or very close to it because attics frequently have a power lines in them and they can carry noise from elsewhere. Sometimes small position changes can make all the difference. I realize that applies to lots of people who can’t put up an outside antenna due to a landlord’s approval or a Home Owner’s Association. A truth that no HOAs or city zoning boards seem to understand is that an antenna causes less hazard to people and their electronics when the antenna is mounted higher and therefore farther from those things.
If your rig is anything other than the lowest entry-level HF radios, it probably has an adjustable Noise Blanker and Noise Reducer function. Even my $25 RTL-SDR has that, in the SDR# (SDR Sharp) software that runs it. Many people ask about leaving NR and NB on all the time. In general, that’s not a problem. I think Noise Reduction is less likely to be a problem. It’s possible that leaving a hardware-based NB on all the time can actually create more noise if you set it too high. This is an excellent video showing the effects of both of the NB and NR in action on a relatively high-end ham transceiver, the Icom IC-7610. There are similar demonstrations with the IC-7300 which is more of an entry-level transceiver and is said to have the same software.
The more modern radios will also have things like an automatic DSP filter intended to eliminate the sound of someone tuning up, but will also work on noise sources that produce a fixed frequency (heterodyne) tone. These operate opposite to the way the Noise Reducers work; they’re correlation cancelers not enhancers. They’re fine on SSB voice and some can eliminate several stations tuning up at the same time. On CW, they have to be tuned a bit more manually, since if they eliminated all of the single carriers, they’d eliminate the station you’re talking to, as well.