2006 Accord - 2.4 - EX L - AT - 115K
My wife and her 3 friends driving to airport (MN) .... smells first resembled manure .... smells would get strong and then subside ... over time it seemed like fumes from something leaking (driving conditions:temp 30 degrees - 65 mph - 105 mile trip to Minneapolis) .... 4 days later return trip was same pattern. I drive car 20 miles to try and reproduce smell .... nothing. There were no warning lights on dash. Been a fantastic car, well maintained , runs great.

 

Sure you weren't smelling something outside the car?

 

Sounds like you went thru some pig and cow poop areas and it was strong enuff in the area to be drawn up in your car.

 

If the cabin air filter has never been replaced, it's more than due. These are known to produce nasty smells, but why they would be intermittent is not clear. In any case, an inexpensive repair.

good luck

 

Go to our search function and type in cabin air filters..I put a video link in their
some time ago for How to change it...
Let us know what you find out...