I recently installed central air in my house. My HVAC guy made couple holes for the return air registers in my exsiting plaster wall which was probably built about 80 yrs ago. Everytime I walked by those holes; there's a distinct smell coming out of it. I'm not sure if I need to be concerned about the air quality(harmful chemicals if there are any???) emitting/recycle through my central air unit. Can anyone help or I'm just crazy even thinkng about this????