Check for obvious signs of damage to the frame and wheels. If none is present then loosen your stem and check that the bolt for the headset crown is secure. Retighten your stem and check for play in the headset. Pull the wheel out and check for loose spokes. Relieve any spoke preload that may exist by applying pressure with your fingers to push the spokes toward the center of the wheel and then away from the center. Loosen and retighten your seatpost collar and the quick releases on your wheels. If the noise persists then loosen and retighten the fasteners on your left crankarm. If it still makes noise then take it to a shop.