We called a plumber who replaced the seal under the commode. For about two weeks the smell was gone, but has come back today. About a month ago we replaced the old septic system with a new aerobic system. Could this have anything to do with the smell. Could the seal be leaking or loose again. Any suggestions would be appreciated.
Thank you

You may have answered your own question. If the new Seal took the smell away, its not impossible the new seal went bad. Does the toilet wobble or rock? Either or I install a new seal and see if that again changes. Better yet did the plumber verify the old seal was leaking. If so then seal is the approach and the plumber may owe you one..