I thought my late model front loading washer was developing a mold/mildew problem due to a musty smell. I was looking at the machine contemplating my expensive-but-terrible investment when I noticed a panel on the lower front, inside were directions on how to clean the drain filter every 3 months. There are 6 in my family so it sees daily use and had been (at least) 8 months since buying it. When I pulled the filter out it appeared to be completely clogged and was followed by what looked like a couple gallons of water. I managed to clean the filter while gagging, after which the mold/mildew smell vanished. I’m quite lucky that I didn’t destroy the drain pump with my negligence and recommend reading the owners manual.
Tl;dr read the owners manual.