How to judge if a laptop charger noise is a safe "hum" or a dangerous "crackle"?

I've read existing questions and answers on noises coming from laptop chargers (e.g. https://superuser.com/a/104860/112639 and https://superuser.com/a/832501/112639 and have learnt that there are two very different phenomena - one serious, one harmless - which cause observably different noises to come from a charger:

• A "hum" or "whine" is relatively normal, and likely an innocent by-product of the transformer using electromagnetic plates to convert current.
• A "crackle" or "sparking" noise is potentially a sign of a failing transformer, and a potential fire hazard. There's a comment that a "charger that generates coil noise especially when under load may be defective" (which I believe means, if the noise occurs when the laptop is connected, but not when the charger is connected to the mains without being connected to the laptop).

So, one type of noise can be safely ignored, the other is very serious and is a reason to stop using that particular charger.

Useful information - but existing questions and answers have focused on explaining the why and have not gone into detail on what to look for when diagnosing and distinguishing the two. I'm looking for some expert guidance on what key distinguishing criteria there are.

In my case, it's not obvious which phenomena this is: the noise is somewhere between a "hum" and a "crackle", so I need some more precise criteria to assess it. These are some observations I've made - I'm sure an expert could judge which of these signs are normal or irrelevant, and which (if any) are cause for concern:

• The sound is a low "buzz" or "fizz", sawtooth sound, a little like crickets (cicadas), or like a lower-pitched constant version of the sound of a fizzy coke bottle being opened.
• It is much louder when the laptop is connected. When the charger is connected to the mains but the laptop unplugged, I can hear a fizzing sound only with my ear very close to the charger. When the laptop is connected, I can hear it from 5 metres away.
• There are occasional random "click" or "tick" sounds. If these were more frequent, they'd sound like a crackle. They happen once or twice every 10-30 seconds.
• It seems to get very slowly louder the longer the laptop is plugged in.
• The sound changes when the laptop is turned on - It stops being constant (like crickets) and becomes quieter, lower pitch, and more erratic.

What are the characteristics of the dangerous crackling of a failing transformer, that set it aside from the benign whining of a safe transformer?

This answer will apply not just to the charger, but computers in general. The focus is on sounds your computer doesn't normally make. There are two kinds of characteristics that differentiate worrisome sounds from benign sounds.

• "Music" vs. "Noise"

  Music has been defined as regular frequencies at regular intervals, while noise is irregular frequencies at irregular intervals. Benign sounds are generally "music" -- uniform and regular; a single frequency, pure pitch. This would include a hum or whine, or a regular, uniform buzz (which can be transformer laminations vibrating, which is normal; other Q&A addresses steps you can take to quiet that).

  An irregular, or non-uniform, sound, or one that contains multiple frequencies that are not harmonious, tends to have a harsh quality that is irritating to hear. It is generally associated with things that are not good. A squealing sound coming from a drive, or audible rotational noise (a "loud" drive), are bad signs. Similarly, sound patterns that change erratically are not normal operating sounds.

• Impulse Noise

  Impulse noise is sound characterized by extremely fast rise and decay and extremely short duration; sound "spikes". An individual impulse sounds like a "tick". A collection of impulses sounds like static or crackling. This is different from a mechanical "clicking" sound. Mechanical parts resonate or "ring", so actions that cause a "click" produce a "rich" sound that sounds "mechanical". It is how you can guess at what kind of material produced the sound. Impulses are sterile sounds. Impulse noise is rarely normal or good.

  A "fizz" sound is basically impulse noise, like static, but it can also sound a little mechanical because it gets carried by physical components. It isn't a normal or benign sound. It could be caused by a capacitor in the process of failing.

  A spinning drive can make mechanical clicking sounds when actuators move, which is normal. Repeated ticking sounds that are in synch with the disk rotation are not good.