Same draw same arrow but louder? if so start by making sure everything is tight. Check for any moving parts that shouldn'tbe. Then starttaking items off the bow one by one, sooner or later you'll be shooting a bare bow. If it's still noisy maybe your brothers bow is just really quiet?

What sight and rest are you using? are the c-clips holding your cams axles on loose to the touch? how about your quiver holder, is it vibrating?
Do both of your limbs look like they have the same amount of space evenly divided between them and the limb pockets inside edge?