If it is a true musty/mildewy smell you may have to soak them to get the smell completely out. I had some towels I forgot in the washer for a couple of days due to an emergency. I washed them and the smell went away. Until they got wet again. I had to soak them to get rid of it instead of masking it. Started the wash cycle with just water and 1cup white vinegar. Stopped the machine after agitation and let it soak for 30 minutes. Drained the machine and started over washing as usual. I don't add it to the rinse because vinegar IS an acetic acid and I want to get it all out.