As mentioned, its probably a dead rat/mouse/bird/spoilt milk/meat somewhere. The flies (usually 'lazy' bluebottle fellahs) appear before the smell gives the game away, and sometimes the smell never appears. Been through this a couple of times, and each time there's been something dead involved. See if you can establish the source of the flies - think of it as a household safari - use a hoover or fliespray to kill all the visible flies and take note of where they start to reappear initially - that's the area you need to check carefully.