Rattle coming from the front--is it from the brakes/backing plate ...

due to regen braking your brake pads and shoes last an obscenely long time, however, every 50K miles you need to lube your caliper slides, anti-seize your caliper bolts, and clean & adjust your rear brake drums. You also need to put some fine sandpaper to the inside of your rear brake drums as well as the shoes where they make contact with each other (aka deglazing).

You or any mechanic can do this and will most likely get rid of any noises caused by trapped brake dust in your rear drums. You need to go to a Toyota stealership every 100K miles (or 5 years if you drive less than 20K miles a year as most americans do) for a brake bleed and fresh brake fluid. Brake fluid is a hydraulic fluid and over time it absorbs water and its effectivity as a hydraulic fluid diminishes over time.

I explained how to deglaze the rear shoes, the front pads can be done while driving, while driving under 50 mph (I use 47mph to be safe) put the car in neutral and stop the car in a normal fashion, such as coming to a red light. If it is wet and/or humid do this once a week. If it is a dry arid season you can do it once a month.

Most brake shops give you a free inspection, if doing the above general maintenance doesn't solve it and a brake shop says your braking system is functioning properly, I would guess it to be a wheel bearing as many Prius owners are forced to replace those.