My basement bathroom, in a 60 year old house, has started to have intermittent sewer smells. The smells permeate the house if not vented out basement windows. All drains flow fine, no backups. The usual things have been looked at: traps are full, roof vents are not blocked, basement toilet has a good seal, bathroom tub and sink drains were cleaned as was the main line. This just started two months ago and three plumbers have not been able to solve it. A friend suggested that the line out to the street (that was just cleaned) might have a crack in it or otherwise be damaged in some way. How can I determine if this is the cause of the sewer gas and what would it take to fix it? I have not done a smoke or peppermint test yet. Thanks for any help.