If its sour/musty, I'd pull the whole thing apart, replace the o-rings and such and make SURE the whole thing is clean and sanitized. Take it from someone who had a nasty lacto infection in a keg. I threw away a bunch of tubing and rebuilt 5 cornies just to make sure it was gone.
edit: I wound up having to dump two batches of beer because if it as well. Please, think of the beer!
Barrelled: Oregon Group Brew Stout
Long term Secondary: Fool's Gold Barleywine
Next: Bohemian Pilsener