I have been having an odor coming from 2 of my vents. It is a burning metallic smell. I have had the service man out twice. He first sprayed the coils inside to eliminate any bacteria or mold. This didn't work, so he put in an odor absorbing block that eventually evaporated in the air return. This has not worked either. The service people say there is nothing else they can do. This is in a unit approximately 5 years old with no previous problems.

Where are the vents located? (the rooms)

The vents are in the ceiling in the kitchen and dining area.

Could cooking be a part of this problem? Are there bedrooms above the kitchen and dining room area? Do you have children living at home? If there are bedrooms above the kitchen and dining room area are they the kids bedrooms?