Hi i was hoping somebody would be able to help me.

I had my fan belt changed today by my local mechanic. I have just realised that i can hear a knocking sound when the car is switched on and when i rev the engine or when i am reversing. The knocking sound is not there when the car is idle.

Would anyone have an idea on what this could be? I will be taking the car back there but just wanted to get some ideas from you guys.Much appreciated.

Thanks in advance