Rattling under parking brake

Sunday I was driving back home from a trip on the highway when a semi in front of me brake-checked me at 70 mph and I saw my life flash before my eyes. I tried to slam on the brakes as little as possible but of course the tires put out the burnt rubber smell. Obviously, this is not at all an idea braking scenario (especially since I am almost obsessive about conserving the most of the regenerative braking and optimizing my gas mileage) but the brakes should not have been compromised just from a hard brake. After the truck resumed moving and I was able to drive my Prius C again, there was a grinding (closer to marbles rattling, like a plastic piece was loose and hitting another plastic piece in parking brake area). I had to get home the rest of the way, so I was able to tell that the noise was coming from closer to the passenger seat side of under the barking brake lever, but I've been scared to drive it since then and getting under my car hasn't shown anything to be wrong. I don't know of any mechanics around who specialize in Toyota or hybrids, but I guess pretty soon I'll just have to settle for another mechanic since I have to drive. Any guesses? It's a 2013, I just bought it used in late August and it's had no problems. I check all the fluid levels weekly or biweekly and have so far done all the routine maintenance (outside of tire rotation) myself. OD is 132k.

 

Update: The rattling only occurs when the engine engages, and only for a few seconds after. It almost sounds like something is loose. I just had my oil changed and tires rotated + fluid topped off (at Jiffy Lube though, so wouldn't be surprised if they didn't top anything off at all because the car wasn't vacuumed as advertised either and they forgot to put my hubcaps back on) The sound definitely is not coming from the floor mounted transmission and I'm still getting the 50-60 mpg that I always get, including highway. I can't figure out what would be underneath the parking brake. The parking brake still is completely functional but it does sound a little louder engaging. I can't tell if the hard brake made something loosen up in the parking brake or if it's just coincidence, but the sound is unsettling and I can't figure out why the parking brake would be affected. Maybe the counter gear? It doesn't sound like grinding so much as low lubrication or a loose part, could even be part of the console if it wasn't connected to the engine kicking in (although on second thought the engine does shake the car more than the battery mode does... Still, it feels like too much noise). The sound is about the level of a whisper and I won't notice it if someone is talking or the radio is on, but I've been driving the car silent every time I've had to (home the first time and then to the mechanic) so I can try to figure out what's up. I travel alone a lot and don't have family nearby so I'm very worried about the possibility of getting stranded, especially since it's getting really cold.