Odors inside the home are very difficult to diagnose. Your first step would be to make sure that your traps aren’t empty. Empty traps allow air from the septic to vent back up the pipe and into the house. You can run water for a few seconds to fill your traps. Another reason for internal odor are fireplaces or woodstoves. When all the windows and doors are closed, exhaust fans, woodstoves and fire places create low pressure in the house which can pull septic fumes through traps. If the odor is in the basement then you could have a venting issue & should call a plumber.