Why does our central air unit emit a very sour smell for a few days after switching from heat to cooling?

Dennis asked:


An air conditioning technician checked the coils and they were clean with no signs of fouling. Adding bleach to pan does not help, Unit and house is 5 years old – all ducts and filters are clean with no sign of mold. Neighbor’s house by same builder has same problem.

Duct Cleaning Atlanta