Its probably gunge inside the washer. If you only wash on 40 degrees and if you use a non-biological powder you'll get this build up of sludge. You might see this if you pull back the door seal, so you can see into the drum. You can solve this by doing service washes on a regular basis. This is a 90 degree wash, with a biological powder and no clothes. This cleans the inside of your wasking machine and keeps it in good condition.
Richard Brooks, December 2007