I can honestly say I have had basically every issue known to man on my cars. Add on my friend's cars, well that's even more circumstances.
I would tell you it like this:
Generally speaking, if the noise goes away after some hard breaking, likely it is the pads.
If the noise is totally intermittent, scrutinize your heat shields as they may be rubbing on ur rotor(s)
If the noise is random I would look at the metal brake shims that sit on the rotor. They can be off center or not fully pressed in and then deflect under heat and cause noises under all sorts of conditions.
Lastly (I think most people re unaware of this if they have not done their own brakes many times), I have seen copious time the oem or aftermarket brake pads are too large by maybe 1mm, and they bind in the brake shoe and do not release and that can cause noises, pad roasting, all sorts of issues. I have ground down 200$ pads more than a hand full of times. IMHO they are a perfect fit but then they powdercoat them or whatever and that is all it takes to bind the pads in the metal sliders and then you just roast ur pads. That happens each time I do my premuim ebc green and yellow pads so yes. Make sure the pads move freely in the caliper and do not stiffen up as you move them through the caliper without the rotor there.
In my opinion, the only thing I have not mentioned is a caliper piston that is seizing, but aside from a visual inspection, all you can do it process of elimination and if all the above are ok then it may be the caliper itself.
Whenever looking for a systemic issue, you MUST ask yourself when/how/under what conditions did the problem start. Only you have crucial data as to the inception of this issue and that is probably as important as being the mechanic that is going to fix the issue.
Cars have taught me that is you pay attention, to sounds and timing and feel, you will likely know as much as the guy who will inevitably fix the car. I have saved engines and transmissions from grenading just by being super aware and cautious. But I am not savy enough to fix the issue myself. So I think we are all guilt of not giving ourselves merit sometimes when it comes to fixing an issue.
Find a mechanic that listens to you, that you trust, and let the cards fall where they may. Or if you have the time and resources, do it yourself by trial and error. But it is tedious when you aren't doing that as a profession so be prepared.
