Intermittent, unexplained odor in house

Who's in the room when you smell it?    Is it all over the house or mainly in one area?

I know for us - our house had a chain smoker and there's one area we didn't paint (closet area under the basement stairs) - when it's super humid - sometimes we smell it.

@jeffl  jamie’s question is a good first one.  Try to isolate its source and any pattern like day vs night, hottest day(s)?, is it only indoors….   How long has this been going on?   Have yu checked the attic?

Any pattern to the way the wind is blowing when there is an odor? (Something outdoors like mulch? Something in the walls/windows/attic/crawl space like mold?)

Maybe you can’t explain it, but can you at least describe it? So far it’s obvious that the smell is 
PSEG…. Gas-possibly 

Plumber…sewerage -possibly 

Pest control agent….cadaverine-possibly 

Could be wafting over from a neighbor’s yard…

Do you have carpets that came with the house, and did previous owner have a pet? Even cleaned carpets can retain a urine smell and release it on a humid day. And/or if they had an intact male cat that marked, the odor might come out on a damp day even if everything was thoroughly cleaned, painted, etc.

Environmental cause from outside the house is another possibility, especially since the odor is intermittent.  Even with windows closed, I sometimes smelled the smoke from the Canadian wildfires last week.  Mulch/manure odors can also waft into the house, especially after a heavy rain. 

Without knowing if others in your house notice it, and time course of the odor being evident:

Olfactory aura - temporal lobe seizure.

Wow!   Thanks for all the suggestions.  I don’t think it’s temporal lobe seizure. No smokers.  No farters. It is definitely in the basement and first floor, not the second floor.  Sometimes even on our deck.  

We’ve detected no pattern for when I happens.  We have definitely ruled out gas.  The pest control guy said dead animals and sewage have similar smells but he checked all around the house, inside and out.  He couldn’t check the eaves. 

Would a dead animal’s smell come and go over the course of a couple months?

Would a sewage smell come and go?  Maybe I get the main drain rootered just to be sure?  No water backs up anywhere in the basement.  

A sewage smell can come and go if the water in any of your p traps has evaporated.  If your washing machine drain hose extends too far down into the drain pipe it can suck water out of the trap when the pump shuts off.  Is there a waste vent pipe through your roof anywhere near your deck?

Something else just occurred to me. We were getting an intermittent sewage-y smell in our basement and my husband was worried about a leak in the air stack, or something like that. I noticed that our low flow toilet in the basement (that we don't use that often) didn't flush all the way and wondered if that was the cause. We started making sure we chose the full flush when we use it, and no more smell. I know you had a plumber in but am not sure if a plumber would necessarily have thought of that. Just worth mentioning I think.

The washing machine drain pipe only goes 4-5 inches into the drain.  The Waste vent pipe is two floors and probably 20 feet away from my deck.

DanDietrich said:

A sewage smell can come and go if the water in any of your p traps has evaporated.  If your washing machine drain hose extends too far down into the drain pipe it can suck water out of the trap when the pump shuts off.  Is there a waste vent pipe through your roof anywhere near your deck?

A third visit from PSE&G determined it was gas.   After a second visit from a plumber. Now to find where the leak is!

We had a similar experience. The gas was coming out of a gas vent outside the house.  Had to have a new regulator installed.