Look around at your floor drains or sink drains. One of them may have dried up. Dump some water down the floor drains or unused sink, shower drains in the basement.

If your laundry is in the basement check the sump to see if it needs cleaning. I had a bathroom with a shower put in the basement and had an episode of sewer gas. The plumbers found when the house was built a trap was not put in the floor drain and was allowing the gas to come in. Now the wash tub has to have a plug in it, the sumps are caulked shut and a cover is kept over the floor drain. This has kept the odor in control. Good luck.

baymee....I can tell you've done work for some big buildings where this is a common problem. What you describe is one of the biggest contributers to sewer odor. A clogged vent also does a number. Bees, birds (mostly from droppings sticking to inside) spider webs, a wayward leaf are common causes.

Is this a new problem ?

It is a recent problem. The washer is on the main floor of the house, and outside there is a real tall pipe that sticks up the side of the house. That is the vent I was talking about. So, how would I go about unclogging it? could I pour drain cleaner down it? It looks like PVC

Just look down it and look for some obstruction. Pouring things inside should not be necessary. It could be a wasps nest, or as I said, a drain that's trap has dried up.

I have the same issue. Only get the sewer smell when doing laundry. Someone commented that it could be that the drain hose is too long going past p-trap but that is not the case. Could it be clogged vent when the smell only happens when doing laundry? Help.

Yes it could. If you can pull out the drain hose and pour a pint of water into the drain, try that and see if the smell stops. That means you have an empty trap as described above which is probably due to a clogged vent. Or you could skip all that and go straight up on the roof, find the vent and see if it is clogged or blocked off somehow.

I had the same problem - not only in the laundry room, but also outside the house. Smelled like a septic system problem, and I couldn't tell if it was mine or my neighbor.

By chance my wife happened to notice that the air return vents (to the ERV) were looking dirty; she pulled a filter out and it was gnarly, totally blocked with nearly decade old dust. I hadn't changed the filters since building the house (I didn't even know that there were filters in every return).

I replaced the filters and presto - the AC system no longer makes booming sounds when it starts & stops, no loud air whooshing sounds when the AC runs, no smell in the basement, no smell outside the house. I think what was happening was that there was an overpressure condition being created in the house, which is well sealed, and it was pushing air out however it could - in this case, through the sewage vent on the roof.

I hope this helps you, because it was driving me CRAZY.