Why does my car engine makes weird noise when idling at the red light?

I noticed my car has been making weird noise when idling at the red light. It only happens in the morning when the car hasn't been warm up yet but to stopped making noise once its warm. What could be the cause of this? Could it be my serpentine belt or water pump? I still haven't replace those yet ever since purchasing this car. My car has 115000 miles. My car has an automatic transmission.