Indesit WD 12X. Smelly water smell?

We have recently moved to our current house and the washer is a built in type. Over the past couple of weeks there has been a smell of stale washing water coming from the washer/dryer. It doesn't look all that old but I suppose that means nothing. Can anyone give me any advice on what to do to cure this horrible smell? the washer seems to be emptying ok. Also what is the black plug type thing for bottom right of the front panel?