A combination of low temp washes and biological detergents encourages a growth of slimy mould inside the drum. Do an empty wash at the highest temperature setting and put a cupful of white vinegar in the drum. The combination of high temp and increased acidity should clear it out. Do that every couple of months to keep it clear.
Might also be an indication of bearing seal failure but not likely in a 2 year old machine.
Check out www.2ndwave.co.uk/manual.html
dannyboy
October 2008