You can add white vinegar to the rinse cycle or put some in your fabric softener dispenser if you have one. About a cup per load initially then you can reduce to half a cup or less if your towels smell better. They may smell like vinegar when they are still wet but will dry and smell fine.
Also be sure to not wash too many towels at once. If you overload the washer it may not be able to properly agitate the clothes and the soapy water. This can lead to smells. Remove the towels immediately after they have been washed so they don't sit and dry them. Drying in the sunshine is preferable, but if you can't sun-dry use the dryer making sure that you don't let them sit in there damp.
Another front loading washer! The sour odor in your laundry is due to water billed up in the bottom rubber seal that seals the door water tight when the door is closed! The gasket should be peeled back a little and wiped down. Water collects here unnoticed and and creates a mold problem that affects the freshness each load of laundry. My daughter and son both have them. It's been a problem for at least 3 years now! My daughter is going to try to leave the door open to let it air dry. Apparently its been a problem on several different models. My son has a LG and my daughter has a Maytag! Good Luck!
Make sure you are washing your towels in hot water to sanitize them. Also you can add the oxyclean tabs inside the washer with the towels as this helps with smells. I also have a front loading machine and always leave the door open when not in use to let the seals air dry. I have not had a problem yet, but I also wipe the inside of the gasket to keep it from collecting water about once a week also.