The ground floor powder room developed a foul odor about 6 months ago. I noticed that the water in the toilet was occasionally slightly brown or gray and that the odor diminished when the toilet was flushed (clear water then filled the bowl). But the water would discolor and the odor would return in a few days (sometimes in a few hours). The wax seal was replaced on the toilet, but the odor return in less then 24 hours. Often the odor is strong enough to make me gag.
Now one of the upstairs toilets has developed the same problem. The water from all sinks and showers is running clear. The 3rd bathroom toilet has not developed the problem (yet). Nothing appears to be leaking. Any ideas what the problem could be?