What kind of insect?
It looks like some kind of gnat. Possibly fungus gnat. I could better determine possibly if you could bring a sample into the office preserved in rubbing alcohol or apple cider vinegar. Where are you finding it?
They have consumed my house and car and me. We had a sewer line break under house and didn't know it for a long time. i know, but it was done by person who fixed it several years ago. There was no smell to alert us until it became a huge problem.
Could they have started from there and when we fixed break they came inside?
That is a possibility but hard to say for sure. They do like moist environments. Here is a link to a publication by one of the UK entomologist, https://entomology.ca.uky.edu/ef632, it might be useful in treatment. The real issue is just eliminating the breeding ground with would be the moist area. Sprays commonly only kill the ones they come into contact with. If you have houseplants they can also camp out in the soil especially if it is kept moist.