House Fly help needed.

[Octoberbride03]

For the past couple of weeks we have been getting invaded by houseflies. We haven't figured out where coming from or what's drawing them as the house is decently clean. We have caulked around the kitchen in places that needed it, and poured boiling water down the sink drains, both of which have helped but not solved our problem. We aren't talking 1 or 2 here and there, like 5 a day minimum that we are seeing. Its driving us crazy. Been in the house almost 5yrs now and we've never had this problem b4.

Looking for any ideas. TIA