Our living room carpet is 20 years old but rarely walked on. We live in the desert and the carpet never gets wet and I am sure the dog hasn't wet on it. It looks like new but has begun to have a "mildewy" odor. We pulled it back and checked the concrete slab, padding, and bottom of the carpet and saw nothing. Clean as a pin. Now another basically unused room is beginning to smell the same. We tried baking soda, white vinegar, and ammonia. No luck. Any suggestions?