We have been getting a foul smell in a downstairs front room whenever it rains for about 8 weeks. The local Council first told us it was a dead rodent trapped in a wall cavity and that it would go away after 3 weeks when the rat had decomposed. It has gone on too long for it to be this now. We have had our personal drains checked for blockages and they are clear. All the household plumbing runs at the back of the house and not to the front. The water company has checked the sewer in the road and sent a camera down and they have said it is clear. We don't know what to do.

The smell is sometimes like a rotten drainy smell, other times smells more like sewage. It only happens when it rains, which is often where we live.

I am a reired highway engineer, not a plumber. The smell of raw sewerage is unmistakeable. If you are not sure, you have to find out somehow. Call a plumber if you don't have a friend or relative that can assist. I would say if the public works people (I assume they smelled what you are referring to) went to the trouble of inspecting with cameras, they recognize the odor.
The sewer can look fine during normal weather but if it is being infiltrated with rain water (very common in older city/county systems) it could be backing up during rain storms. If that were the case, you should have heard from your neighbors too. Can you smell it outside --near the street?
Could it be a toilet gasket? Could there be tree roots causing damage to the sewerline from your house to the street. Maybe the rain association is coincidental. Do you have a cleanout on your sewerline you can remove. If you do removing it will give you a dose of the unmistakeable smell referred to earlier.
Not knowing the terrain around your house and type of system your town has I would still think it is unlikely that a backup in the mainline is causing a problem because it would be very unusual if it only affected one house unless there are no other houses in the area.

Comment: We get a foul smell in our bedroom when it rains. The smell is not in any other room. We took the room apart and find nothing. We too thought it might be a dead rodent in the walls, but it has gone on for about a year. The smell is not outside but I checked the gutters on the roof for dead rodents. The house was buildt in the 80's and we do have problems with the plumbing. The toilets get clogged very easily. Had plumber replace toilets but still get those problems. Every once in a while the entire house looses water pressure. Someone told us the water main valve and meter is under water. Don't know what that means. Still trying to figure it all out.

You know I keep hearing the same things/reading the same things about it being a dead rodent in the wall... geez I know that dead animals can smell sometimes real bad too... but I've never smelled something this bad in the one room and in the garage... actually the whole downstairs!! When I leave the downstairs and come upstairs there is no smell. IT ONLY HAPPENS AFTER IT RAINS! THIS IS HOW WE NOTICED WHEN IT CAME AND WENT AWAY! IT TAKES LIKE 2 DAYS AFTER IT STOPS RAINING TO GO AWAY. We thought that it might be a dead animal in the gutter... but we cannot check that ourselves... I also think our gutter is closed all the way around so I don't know how that would be the answer anyway. We also have problems with our plumbing too... toilets get easily clogged... especially downstairs, where the smell is. And the upstairs 2nd flr. Toilet is bad too. The 3rd floor toilet flushes fine though. I wish someone had answers to our questions! We have no idea where to start... rather where to check next...

I have had the same problem. They finally found dead rats under the house and I had that cleaned up. However the smell still came back despite the removal of the carcasses. The next step is to have someone disinfect the area to remove the urine and droppings which we believe develop the smell when humidity/water comes in.