Hmmm... I just went through something similar a few weeks ago. The thing about that type of odor/gas is that it will rise and linger depending on air currents in the room. You may not be smelling it near where the source is. I will tell you that with our shower, we have been squeegee-ing the walls, and rarely use cleansers in the shower, and I have lots of thick hair that can get stuck in the drain (but not clog it) and what I found was that after I had cleared a big hair clump, the actual drain trap was stinky. I had been smelling it for a week or two- it smelled like sewer gas; I poured 1/2 cup of Clorox into the drain trap, let it sit overnight, and now is 3 weeks and haven't had it again. I think my trap simply got funky. Another time a plumber didn't put the house trap on tightly and one corner of the basement was where the gas collected... 10ft from the source. So look around, not just where the odor is, cause that kind of odor is rarely sitting right at the source! And in the event you haven't been using that shower, make sure there is water in the trap (which prevents the sewer gas from rising out of the drain).

Your trap is dry. The forced air causes the water to dry rapidly because of the pressure imbalance. This is a common problem in commercial building lavatories where the exhaust fan runs constantly. A solution is to add a fresh air intake vent in a basement window to balance the pressure. Another solution is to completely seal off the basement from the climate controlled zone of the house, but it sounds like your basement is part of the conditioned space of the home (most likely through the old floor boards). A third solution is to put water down the floor drain every day. The hole you see in the side of the floor drain is probably a tee that runs to your foundation drain system.

1. Smells occur ONLY after a rain and are very string OUTSIDE on both ends of the house. "Both ends of the house" are you referring to the front of the house and the back of the house? Is the front of the house where the main drains to the sewer? 2. Odor does come inside and seems to be most highly concentrated in a second floor closet, not near any vent or plumbing. There is no air duct into this space. Baffling. 3. NO smell in attic ok 4. Partial basement then crawl space. No leaking in crawl or smell in crawl. Everything appears OK. 5. NO SMELL coming from any drains or in any bathrooms. Makes sense, each fixture has a p trap 6. Ran water down 4 roof vents. One is blocked. It services one bathroom. This bathroom never smells. Plumbers cant get the snake to go thru. May try high pressure water treatment. Probably not related. 7. Worst smell is at the opposite end of the house. As with #1 above, is the opposite end of the house where your main goes to the sewer? Is there another house at this end of the house?? Have they had their septic converted recently also? 8. Reminder, we never had a problem while on septic system. What time of year was the system converted? My first thought is maybe you have a pipe separation that occurred after install in hard, dry ground and separates/separated when it rains and the ground swells. I don't think it is a coincidence that after the convert you smell gas - something is wrong with the conversion. The smell you get inside is hard to fathom not knowing the layout of your house. Seems very strange that a interior (?) room on a second floor with no plumbing, HVAC vents (true?) or windows would smell like sewer gas.

Thanks to everyone for all the comments. I truly believe that I have clogged roof vents. Won't know for sure until I get someone to snake them and see if it takes care of the problem. The plumber that just came out told me that he thinks I have a cracked/broken pipe and that he would return when his camera is working so he can scope it out. I honestly do not believe that I do and I'll find out if my theory is correct once I've had the vents cleared. I will only pay for him to "diagnose" as a last resort. From what I've learned doing research and speaking to people with the same issue, the most probable cause is a clog. Also, the plumber did change the wax seal but that didn't stop the water from coming up. Will be replacing it again when my issue is resolved cause water has been coming up through that wax seal. I'll definitely update when I resolve issue. Hope my experience can help someone else.