Loud whining sound when I accelerate

Four cylinder front wheel drive automatic.

Guys, All of a sudden is making a "whining" noise (similar to a cat) when I accelerate at any speed. The mechanic is not sure whats going on with it. Have you come across this sort of thing and if so what could the problem be? It is not making the sound in idle. Only when the gas is hit. The sound is coming from the back half of the engine. He thinks it could be the alternator belt or a pulley but still not sure. Also, My car has a four cylinder turbo if that makes a difference.