During a week's hiking around the High Sierra Cps of Yosemite, my backpack began to reek! Even though I washed myself and my clothes regularly, my husband, who is normally a very sweet man, insisted I hike behind him so as not to be asphyxiated. How, oh how do I get the stench out of my backpack straps?
Ah, the heartbreak of smelly gear. And it's a common problem. All that sweat, moisture, and dirt that's such a big part of hiking creates a perfect environment for bacteria. And that is likely your hiking aroma culprit: A severe bacteria outbreak in your pack straps, and probably in the backpad as well. A little colony of germs, busily munching on sweat particles, little flakes of skin, shards of dirt, and heaven knows what else, all the time kept moist and warm by the proximity of your sweaty body (don't take that too personally, I'm sure your sweet husband is most enamored of your sweaty body). And as those little guys munch, of course they leave their mark. That smell is one of them.
So what to do? I don't think just washing the pack in soapy water will do the trick, although it might help. I think you really have to kill the bacteria. What I would do is get a bottle of Lysol, fill a tub with very warm water, dump in the Lysol, then throw in the backpack. Let it percolate for up to half an hour. Then, empty the tub and re-fill with water. Let the pack sit some more. Then empty the tub again, re-fill, and add some Dawn or similar liquid detergent. Scrub the pack with that. Then rinse thoroughly and air-dry completely.
That oughta do it. If it doesn't, well, tell your husband to hike with a nose clip.