Odd metallic chafing sound
This could be coming from the bottom bracket, which will make noise when worn out and pedalling under load. One way to test this is to stand beside the bike, grab one crank in each hand and see if they can be rocked towards/away from the frame. There should be no play in this direction. This is described with pictures here.

If that's not the problem, then the following link has a rather comprehensive list of possible causes of bike noises.