Forensics...

Build up of bacteria
Smells in a washing machine can be caused by a build-up of bacteria and other deposits. Dirt removed from clothes can remain inside the washing machine creating an ideal breeding ground for bacteria. In the dark, warm and damp interior of washing machines, fast breeding bacteria can begin to release unpleasant smells. In extreme cases these smells are transferred to the clothes in your wash.

Hide the evidence!

A maintenance wash requires a hot wash
(60°C or more - 90°C recommended) once a month with no clothes in the machine and with a bleach-containing detergent such as Ariel Biological Powder.

Other maintenance steps may be required as sometimes mould can appear on the rubber door seal (particularly under the rim) and this might not get washed off during your maintenance wash. All you need to do is to simply wipe around the door seal (inside and out) with a damp cloth, then run your maintenance wash as usual.