"slowly letting go of the brakes makes this low hollow sound for a second. It's almost likcue if you step on a creaking hardwood floor"
I love that you compared this noise to a "creaking floor", because in the auto-industry, this is literally called "creaking", for example this article goes through several noises that brakes make, and classifies them as hissing, grinding, squeaking, clicking, rubbing, creaking and knocking.
The currently accepted answer is far from complete. You will see in this answer that I've provided more than a dozen references (including input from ~100 people), and only one of them associates the "creak" noise with the "friction" described in the accepted answer (that was in this comment3 in which the term "friction creak" is used). Also, the accepted answer makes it seem like hearing this sound is inevitable when slowly releasing the brakes, but as you'll see based on the ~dozen references below, that's far from the case.
"Creaking" from the piston-seal interface
As you can see in this article and this article, "creaking" comes from the piston-seal interface (by "seal" they mean rubber boot, dust boot, dust cover, or dust seal, which are all synonyms). This can be solved by adding lubricant on the sticking caliper piston.
"Creep groan" due to various possible reasons
However, as the title of this thread shows, what you're hearing could either be "creaking" or "groaning" which is called "creep groan" as explained here4 and in the title of this. The "creep groan" that you hear when you slowly "creep" forward without the brake released quickly enough, has actually been discussed on this site many times, for example:
If the reason for the noise was merely as simple as an inevitable friction between the pad and rotor, you likely wouldn't have asked the question because you'd hear it all the time. It turns out that most people ask this question when they start hearing this noise under certain conditions1,2 such as reaching 65,000km, at certain temperatures, in wet weather, idle time, etc. Many of the references below will contrarily say that all cars make this noise to some extent, but the reason people keep asking this question is because it's louder than normal. The following factors are blamed for the noise:
Blame it on the brake pad material
Most people blame the brake pad material:
- In this case, simply replacing the pads reduced the noise to the level that a brand new car at the dealership had, which was still louder than what we're normally used to (so part of the blame might go directly to the model of the car being a Mazda Gen 3).
- Here the use of aftermarket pads/rotors was blamed. See here for the full thread.
Blame it on the brake retaining spring or brake shim
- Here it's suggested that the noise can be reduced by replacing the brake retaining spring or brake shim, see footnote 2 for the full thread.
Blame it on a bent dust cover
It was suggested in the first response here that a bent dust cover could be touching the rotor. This is could be what the problem was for me, since I had a ripped rubber boot on the piston, which was likely touching the rotor. By replacing the piston and rubber boot with a new one that was "flush" with the end of the piston chamber, the noise I'm getting when braking at low speeds is much less pronounced, although I can still force it to appear if I want (I'm currently blaming the rest of the noise on the brake pad material which seems a bit metallic and slightly dented from trying to squeeze it in place due in part to the previous piston with ripped rubber boot sticking out a bit, and on the rotors which are not only old but also very dirty because we didn't clean them at all after touching them a lot with gardening gloves that were degraded by brake fluid). By the way, the link in this paragraph leads to a thread in which the noise went away just by changing the caliper.
Blame it on uneven wheels
This is suggested here and could very well have contributed to the issue for me, since in my case I started to notice the noise after doing the brakes in July 2023 on that one wheel mentioned in the section about blaming it on a bent dust cover. Basically I was driving with 3 "new" calipers and one old one, that had a ripped dust cover, dented pads, and a very dirty rotor (though I'm not sure about the difference in how well things were "clamped" as the comment in that link suggests). The noise for me could have been there last year and earlier too, but I only noticed after July 2023 because I was listening for it, since I knew I had done a rushed job that time (actually I only started driving it in August 2023 and didn't drive it much in that month, so September is when I first started noticing this noise).
"Clunking" due to various possible reasons
Another term associated with noise when braking at a low speed is "clunking" for which this article provides 7 possible causes:
- Worn out control arm brushes
- Loose lug nuts
- Worn out wheel bearings
- Faulty steering stabilizer
- Failing end links
- Faulty ball joints
- Damaged shocks
We have now seen that the noise could be due to one of a plethora of different things (although most likely just the choice of brake pad material), but you might be wondering what the consequences are. Despite reading through over a dozen references that I provided here, I have seen people over and over again saying that the noise is not a safety issue, but just "annoying". A brake engineer here even goes as far as to say "the better the performance, higher the likelihood of these issues cropping up".
To fix or mitigate the problem, apart from changing pads, rotors or calipers, four suggestions are underlined in blue here (changing the brushings or suspension system, or lubricating the piston-seal interface, or using 40-grit sand paper to smooth the brake pads).
1 Here is the full thread.
2 Here is another example in which someone only experienced the sound after about 10 years. This full thread can be found here.
3 The full comment thread is here.
4 Here is the full thread.