That isn't fruit fly behavior. We need a positive identification. Catch some, dead or alive but not squashed. Take them to a county extension office, local pest control company office (keeps the sales guy out of your house).
Based on the location you describe, I suspect that you may have "sewer flies" more correctly known as "psychoda". Look that up on the internet and compare with what you catch. If these are what you have, then a cracked sewer pipe, or stagnant water or partially clogged drain may be present.
Get the identification first, then we have a good direction to go. Keep us posted.