Comment by anticodon

I had the same problem. After unsuccessfully trying several garages to fix it, I learned how to fix cars myself (read lots of blogs, watched video on YouTube, read a couple of books) and redid the whole brake system: disassembled the caliper, cleaned everything thoroughly, changed brake pistons, changed sealing rubbers and dust covers, installed new brake disc and brake pads, new brake pin (greased it with the appropriate grease). Problem went away.

It was a rusty/incorrectly installed brake pin. Sometimes brake pads wouldn't return after you stop pressing the brake pedal. After brake system rebuild everything moves smoothly and brake pads always return to the initial position - no more weird sounds.

Also, when changing the brake pads, I've noticed that one of the pads (that was failing to return) was much thinner than the others.

Problem is: when they change the brake pads in the dealership/garage, they don't rebuild the whole system or grease the pins properly. You will not notice and they're not paid for the extra quality work. So things like this happen quite often.

The only way to do everything properly is to do it yourself. In case of a brake rebuild it's not very difficult but very time-consuming. Most difficult part was to insert the new piston back in the caliper.