HVAC intake? I don’t think new air would be introduced unless the system contains an HRV or ERV. Additionally, this sounds like improper venting, which is a s

HVAC intake? I don’t think new air would be introduced unless the system contains an HRV or ERV. Additionally, this sounds like improper venting, which is a surprisingly common occurrence. So the gases could be coming up from the drains and toilets.