Just read your other post, and you said it had been a week between uses. When you said, "dirty," could that be the same as "musty"?

I have, with other filters, noticed that if I didn't clean them and let them dry, and there was some time between uses, that I would get a bit of musty flavor in the first quart or so - kind of like that smell you get when you go down into an older basement that isn't well ventilated. However, it was never very strong and didn't last beyond the first quart.

It could be a slight bit of mold or whatever crud starts to develop if they aren't dried out. Go ahead and backflush with the bleach-water the instructions on the bags indicate, and see if the taste is there on the next trip. Just remember, after you backflush with the bleach water, backflush with the air-filled syringe until no little dribbles come out the other end of the filter - that means you blew out all the water you could.

One other thought: what are you filtering the water into? You could have the same musty-taste being generated by the container, not the filter. Did you store your bottles closed and wet? If you were using a bladder, did the hose get disconnected, cleaned, and dried before storage.

And then, there's always the obvious: do you store your filter or containers inside your socks? (Sorry, too good to pass up.)