All summer we've (well mostly I) been having trouble with smelly, mouldy towels. Didn't want to use bleach because these were the kids' towels and they were brightly colored, also bleach seems to leave my towels a bit stiff feeling. After a bit of surfing, ask Dr. Google, I found a suggestion to use white vinegar in the rinse water to get rid of the stink. It works mostly.
Don't accidently add it to the bleach dispenser instead of the fabric softener dispenser.
It adds the vinegar in with the soap, not the rinse water.
Resulting in lots and lots and lots and lots of suds. So much they were coming out the soap dispenser.
Also, don't then add the vinegar to the fabric softener dispenser in addition to vinegar you've already put in.
Three or maybe four cycles later my towels are really clean.