What to do about a smell in home of septic when running washer?

If the PVC pipe isn't capped with a screen or something where it comes out the roof...maybe there is a dead bird stuck in there and you only smell it when air is being forced up that pipe and can't go through. Like when the washer is going.

Installing a fan in the vent pipe can cause negative pressure and suck the water out of P traps. You need to make sure the roof vents are clear. Kitchen, bathrooms, and utility rooms have separate vents. How does the washer drain?

Turn off the washer water, pull the washing machine away from the wall, mop and dry the floor, put down a layer a paper towels, and leave them for a couple of hours. You should be able to see if there is any leakage coming from a nearby refrigerator, sink or toilet that might be sharing a wall with the washer. Check your other appliances similarly. Pull your drain hose and water lines to the washer as much as you can. Run a straight wire clothes hanger into each to see if you need to clean out and hoses (like from a sock or detergent buildup). Have your septic line cleaned out. Use Drano on every line. If any lines are sluggish or slow, that could be your culprit. Have one of those plumbers make sure none of your septic hoses has a leak or disconnection. Pour bleach through every line you can. Those are my ideas—see if one of these helps. ☺️

I'd avoid using Drano or bleach in a septic system!

Just a thought...Perhaps your washer itself is dirty. They can build up debris on the outside of the washtub that you may not see, particularly front loaders. There are commercial products for cleaning this.

https://www.plbg.com/forum/read.php?1,416919

We chose to install a dry well for our washing machine and it's worked out really well for us and eliminated any prior problems. See below:

https://www.hunker.com/12002471/how-to-create-a-washing-machine-dry-well