These are flies, but I can't make out what type fly they are from the photo. Flies can be found in moist areas such as drains or feeding on organic matter or dead animals. 

I would recommend covering drains overnight with a layer of cardboard cut-to-shape and covered with petroleum jelly. The next morning, see if you've found any flies on the board. If none are found, then the issue could be from flies living in a wall void or feeding on a grain or pantry item. 

You could also try hanging a few yellow sticky cards to passively attract and kill them.