Our new machine has a cycle ( setting ) for Drum cleaning, conveniently its called " Drum clean "

BUT

Our last one that was 10 years old started to smell a little and one wash churned out this grey sludge ( all over a white wash which wasnt great ) anyways, i spent some time looking into it and the popular answer was 1 wash ( of nothing important, we used dog towels ) " a hot wash, use SODA CRYSTALS instead of detergent ", this flushes out everything

it worked well, we used to just wash the dog towels on a Hot wash with soda crystals, the smell and great sludge stopped

By all accounts old washing detergent used to contain soda crystals, but they stopped a while back