I've had cheaper pedals go after that amount of riding and make a sound like you mention. You can try a new set; if the noise goes away, there it is. I've also has a large saddle bag (Carradice Super C) push a brake cable in each pedal stroke (older center pull brakes) which caused a nut on the brake cable "tick" against the rear reflector mount. It drove me nuts till I figured it out and removed the reflector mount. Now no noise!