have you cleaned out the filter? all the fibres caught in there may be going mouldy.
empty that then put the machine on a quick hot cycle, with some powder etc in, but empty, this will give it a good rinse through.
if its still there, you could try a spray bottle of bleach and blitz the interior - but be sure to do a empty cycle before use - otherwise you may get bleach patches on your clothes
another thing could be the door seal - sometime icky stuff collects there in the folds - give that a wipe over