We have flies coming out of our AC vents. This has been for the past 4 days. After the AC goes off, several flies some out of the vents and fly up to the
Indow. They are very weak, and am killing them at windows with a paper towel. We cannot figure out where they are in vents and why. Can an exterminator help and what can they do?

Usually flies have laid eggs in a dead mouse or food somewhere in a duct.
Yes, exterminators can find the source.