Septic tank odors are coming from the air conditioning vents. Air condition systems do not create septic odors however; they are able to recalculate septic odors making it appear that the odors are worse then they really are.
This is usually caused when a sewage drain line or a septic drain field is exposed and leaking toxic effluent to the environment. The air conditioning ventilation system will on occasion pick up septic odors sending them through the system spreading throughout your home.
Anytime septic tank odors of smells are obvious, it is important to contact authorities and your local plumber immediately. Turn on fans open windows and doors in an attempt to allow fresh air to inter the home displacing septic tank odors and gasses. Evacuate family and pets immediately until the problem is resolved.