It is most likely that the U-trap that provides a water trap to separate the foul air in the sewer from the washing machine is being syphoned empty of water. This can happen if the pipes are getting partially blocked with crud from the washing machine powder. Need to try something stronger than soda crystals (drain cleaner) if the above idea doesn't work.
If the previous two good advice posts don't help just try leaving the door open between washes. That cured our problem. Seems the tight seal causes some bacteria growth in the remaining damp areas if you don't use it everyday...