Your best option is to provide a sample to your local University Extension office or your Department of Public Health for proper identification before a responsible treatment can be offered. There are many dozen kinds of small flies, midges and gnats that could be the culprit. Depending on the species, there are other places besides he drain that can host a population. Houseplant soil, high levels of humidity in or near the A/C equipment or bathrooms, etc. Running a dehumidifier in basements and crawl spaces may help. Once the heating season arrives, the home will dry out and the problem may go away. There has to be an area in the home where these insects can survive and grow their numbers.