Drum clean washes at 40°C if not mistaken.
This could be accumulated over the life time of the washer by the previous occupant.
If DIY, fill up drum with cold water until it fills the inner part but not to the brim. Spin it just fast, if water comes out reduce amount. Remember this amount of water used.
Then boil the same amount of water filled as above but use distilled or RO water in a stainless steel pot...wait a few minutes for the temperature to drop to 90°C and fill'er up with a smaller pot with long handle or brand new kettle that's not been used yet. Spin the drum from the top side by hand with cloth so as not to scald your hand. Spin clock wise and anti clock wise really fast and slow but careful not to spill it out until it has cooled to 70-80°C Then run a rinse and drain course. Do be careful when transporting pot to laundry area. Ensure kids or other ppl do not hinder traffic to prevent scalding accidents and the floor is not slippery or oily.
The other option is run the machine empty (no clothes, no detergent) at Cotton at 90°C with intensive wash option but you had to terminate before the final spin or even easier - if selectable, toggle no spin option. If water remains, run a rinse and drain course. Try 3-5 rounds and see if soapy residue has decreased.
Avoid or minimize softeners and use low side detergent like Kao Attack or Max Kleen 9.
thanks for your detailed explanation.. but "self-clean cycle" doesnt do all those u listed?