Yes metal, but any hard gritty material can also do this, sand, very small pebbles/rocks. Metal, sand and rocks are really bad for the wheel so check to make sure that the pads do not have embedded foreign material.

Other sources of screeching is old pads that has hardened over time or glazed pads. Glazing usually happens when your ride lots of hills with lots of long downhill. When you have an old pads or glazed pads, the braking doesn't work very well and you have to really brake hard.

Another source is that the pad has worn unevenly and is not toe'ed in correctly.

Last one I can think of is slightly loose bolts?