You might also try this question in the plumbing forum...

I've had septic for most of my life, and never had odor issues when it rained.

It would also help them to know (perhaps) if your toilet is the only one in the house; or a basement toilet; or... anything about it and its location.

I've lived with septic all my life and never had an odor when it rained problem. The only time we've had an odor problem is in the guest bath which is seldom used. If the traps aren't used occasionally and the water in the trap evaporates, you will get sewer gases in the house. So on occasion we run the tub and the shower just to make sure the traps are full. You should check with the plumbing guys...it may be something with the way the toilet is vented that is causing it to back up.

"Does this mean the septic system has a problem? We were told that it does not."

who told you that?

when was it last cleaned out?

how large is it and how many people in the house?

how many in prev owner's family?

I also might suspect a venting issue, before a septic problem. We have an older home and the vent stack is directly behind the wall the toilet is on. It's an external vent stack that terminates below the roof overhang. That alone has caused sewer odors in and around the bathroom when it's damp or very humid or raining. We had our septic system checked and it's fine. We're going to have the vent stack taken through the soffit to clear the roof when we do some additional remodeling.

Also, as Annie said, make sure your traps are properly installed and filled with water. A plumber would be a good call to check all those things.

kirkhall: We have a bathroom downstairs, and this bathroom is on the main floor. The downstairs bath doesn't have an issue. It's between two bedrooms, and is on an outside wall. Thank you!

It's possibly nothing to do with your septic tank, it may just be a bad smell. If it's on your main floor, (ground level?) my first guess is you have a roof or downspout leak, and possible mould issues, being an outside wall. I'd have someone look at that, especially since it's only in one bathroom.

You might have a leaking or broken sewer vent - but I don't know why heavy rain would exacerbate it. I'd also be concerned if there IS a smell with rain and a septic, it might mean you have an excess of rain water running into your septic, not good. Where does your stormwater drain to?

You should probably have a plumber or a competent building person take a look, or both.

Late to the game here but here's my take....

I'm on septic too and it's old but works well. The house is old too, predates indoor plumbing, so when plumbing was installed only the upstairs bathroom was vented and not the downstairs halfbath. When we get heavy a prolonged rain (4-6"+ over a week) the unvented downstairs toilet will burp when the upstairs toilet is flushed. To me it appears that the heavy rains temporarily saturate the old drain field which causes a back-up of waste in the septic tank. This waste back-up covers the outgoing pipe located inside the tank with waste water so when the upstairs toilet is flushed the resulting compressed air in the line has no where to go except to burp out via the first floor toilet which is not vented. The odor you smell results from the burped/expelled air.

I hope my "theory" makes sense as I am no way an expert on this topic. But in my case it seems to be the only logical explanation due to heavy rains and a saturated septic field. My only real concern is that too much toilet flushing, showers/bath use, laundry use, etc during these rain events would back up the waste water all the way into our house via the lowest drain outlet.
I'm open to any thoughts on my thoughts about this, would like to know if it's "normal" for heavy prolonged rainy periods to cause old septic tanks to back-up?
Thanks