To repeat one of sharkman's questions - what does the noise sound like? Is it a hiss? A hum?
If it's a hiss, it might be caused by the amp, the Yamaha, or a combination. You could plug the speakers into the amp, disconnect everything else from the amp (especially the connections from the Yamaha) and see if the hiss is still there. If so, it's the amp. If the hiss only happens when the Yamaha is connected to it and turned on, then it's caused by the audio outputs of the Yamaha. If there is a hiss when the Yamaha isn't connected to the amp, but the hiss gets louder when the Yamaha is connected, then I'd say it's caused in part by both components.