BLACK or SLIMY MOULD IN MACHINE, or AROUND DOOR SEAL: This is a fairly common problem, where either a musty odour comes from the washing machine or washer dryer (especially after a lot of low temperature wash use) or there are dark or black mould spots on or around the door seal, that can get on to clothes. There is no component that can fail to cause a bad odour from inside a washer. In most cases, it will be from some external source, or the way the machine is used.
The major culprits are the low temperature liquid detergents, especially the BIOLOGICAL ones, as these tend to promote mould growth, especially if the door is left closed after washing. The combination of this, plus using a liquid softener will also promote soap sludge and mould over time.