I just had the fluids changed on my bike at the shop, and I hearing a strange sound coming from the front break. It seems, for lack of a better description, like one of the pistons is acting up. The breaks work, but I'm wondering what could make this strange sound that's hard to describe. They don't feel spongy, so I'm at a loss as to what would make the breaks do this after the break fluid was changed. Any ideas? Thanks.