silverado

Ok, I'm out of town and my wife calls me with this problem. She says that the car idles fine with the brake pressed. When she realeases the brake the car vibrates strongly and acceleration is very poor up to 40-50mph. Along with this is a clacking, bird chirping (her description) type sound. This has happened before, but the car will go a couple of weeks between it happening again. The car is a 05 AT. Any help is greatly appreciated.