Why does my car make a weird sound at low speeds? sounds like crunching metal

What is your question?

My car runs fine and has only about 70K miles on it. I've noticed that it makes weird noises when i'm sitting in heavy traffic, with lots of stops and starts. mostly when i press the brakes, it almost sounds like a crunching metal noise... Brakes work fine otherwise and i don't notice the problem when car has been running at normal/high speeds.