The smaller the vibrating object is, the higher pitch the squeal.
As I understand it:
really high pitch squeals are caused by the pad corners (chamfers may fix this)
high pitch squeals are caused by the pads
medium pitch squeals (under 10kHz) will be the calipers vibrating
low pitch squeal/graunch is when the whole upright assembly vibrates
Car manufacturers add mass (more weight) to the calipers to tune out the squeals, so just be happy that AR/brembo have not done this as the car is heavy enough already...