Mazda3 making a loud noise when backing up.

[AzNightmare]

I first started noticing this issue around May.
The only problem is, it happens so randomly, and not very often. Maybe about once a week.
And it could happen any time, cold in the morning, or when it's warmer after a 15 min drive and I'm parking, etc.

Just about 10 minutes ago, I was able to replicate the noise 3 times in a row in my garage,
but as soon as I took out my phone ready to record the audio... it stopped happening.


This is why I'm hesitating to just take it to a shop, since chances are,
I wouldn't be able to replicate the issue once I'm there.

Basically, when I'm in reverse and backing up slowly, foot on the brake pedal and releasing slowly,
I could feel a very faint rubbing/grinding noise. I'm not sure if its just coming from the brake pads and if it's normal.
At this point, it's very faint and not even audible anyway, unless you're really listening for it.

Then out of nowhere, a loud noise comes out, and I'm not sure whether it's from the wheels or something else.
It sounds similar to a squeak on bad bicycle brakes, but it also has a low hum like a foghorn pitch.
And the sound feels more like a "vibration" type of sound. (Sorry, that's the best I can describe it with words)

I recently had my brake pads changed at a dealer earlier this year, so they are pretty new.
Not sure if it's the disc brakes, but it only happens in reverse, and when it does happen,
the sound seems to amplify or cease depending on how much pressure I release off the brake pedal.

Hope someone can throw some idea/suggestions, and maybe help me narrow down the issue.

[Phil@rise]

sounds to me like the pads shifting slightly in the caliper