2004 Dodge Stratus Heard Heavy Knocking Noise Under Hood Right Side Car

I heard a heavy knocking noise under the hood from right side of the car where the valle.While driving the noise increased heavy ,its sound like metalic and i stoped driving. .When mechanic looked he said that the motor parts are broke and robber band as well.I really dont know what he talked about bcoz my friend took the car to mechanic. He said that its can be replaced. I wonder if its right,can its be replaced and how much around about its will cost.