It wouldn't be the caliper as it is a "fixed" mount. It could be a sign of the caliper sticking though. The "rattle" is probably a piston not extending or extending unequally allowing the brake pad to vibrate.

Lift the side making the noise, remove the wheel, have a helper press the brake pedal and check the pads both with pressure for both of them to engage tightly and without pressure to see if they both retract evenly.

It could also be a hard spot on the rotor. Did the problem occur suddenly or have you noticed it for a while?