The trouble with gulls is that they are moving inland in huge numbers, larger numbers nesting inland than on the coast due to easier pickings of food. The problem you are having is most likely caused by the parents of young gull that are on the ground and still learning to fly so mum is screaming and shouting as a warning, usually first thing in the morning and probably nesting on a rooftop nearby, the problem should go very soon when youngsters are able to fly and leave the nest. Very unlikely to be caused by drains.
im a pest control officer