I have an infuriating intermittent squealing in the front of my 2016 Mazda 2. It's been going on 3 months now, and is getting progressively less intermittent.

It's an inconsistent (as in not a constant squeal, more of a rapid series of short squeaks), metal on metal sound. Typically it only occurs at lower speeds (under 60kmph/40mph), and stops immediately if I tap on the brakes (only to start again if I let go). I've had it into a mechanic, who resurfaced the pads and pulled a pebble out from between them, but to no avail. He said they still have a good 50%+ life on them.

The brake pads & calipers were replaced 1.5 years ago.

Any ideas?