Don't squeak when braking forwards, or braking hard/fast backing up. Only when backing up and lightly pressing on them. Extended squeal, and very loud.
Anything I can do?
This sound usually comes from resonance when pads sharp edge touches the disc slightly or a small piece of stone stuck on the pad. Usually it's not serious and will go by itself after braking several dozen of times, but if you want to be safe - get the car to the service-shop and ask them to take a look and maybe grind the pads sharp edge.
Other more serious options, that probably do not apply to your description, as they would produce sound at both directions and range of braking forces: