Are you sure your washing machine is going to your septic tank? Mine goes to a separate drain than toilet, shower, sink etc. Now my house was built in 65 so that could be part of it. I also have noticed two damp spots on the ground in my gravel driveway. Need to dig it up to see if it’s the main waterline coming from well to house(doubtful since no real noticeable lack of water pressure) Or drain field which supposedly heads that way. My washing machine stays backed up. Think it has to do with amount of water being much more than toilet or shower