Our house is a 3 year old split entry with the laundry room in the basement. In the last 2 months I've noticed a strong septic odor coming from the laundry room but only once in a while... maybe every 3 out of 7 washes. The septic tank (1000L) was pumped in September and this is the only area in the house where the smell is located. The smell, when it comes, disappears after a few hours. We have all the proper plumbing connections, p traps, etc. I've tried putting vinegar in the washing machine (full cycle, hot water) but the smell keeps returning. We do have a vent on the roof of the house, it is all up to code and isn't blocked. Can you think of anything else that it might be?

What else is connected to that line?

Our house is a 3 year old split entry with the laundry room in the basement. In the last 2 months I've noticed a strong septic odor coming from the laundry room but only once in a while...maybe every 3 out of 7 washes. The septic tank (1000L) was pumped in September and this is the only area in the house where the smell is located. The smell, when it comes, disappears after a few hours. We have all the proper plumbing connections, p traps, etc. I've tried putting vinegar in the washing machine (full cycle, hot water) but the smell keeps returning. We do have a vent on the roof of the house, it is all up to code and isn't blocked. Can you think of anything else that it might be?
There is a laundry tub that is connected as well...

I sat here for 20 minutes pondering this. The only thing that could smell would be 1- a spill, 2- a backup, 3- no trap, 4- a leak inside or behind the machine, 5- a dead critter, or a clogged vent. Other than that I am not thinking of it right now.