Try putting some bleach into all the detergent tray and the conditioner trays and running the machine with nothing in on a very hot wash, then run it again just with water to flush oput any trace of bleach.
It might be that your machine has various little u-bends of pipework where 'grey' water can accumulate, causing the pong.
Also, see if you can check how it is connecting to the outflow. If it's been connected directly to a soil pipe from your bathroom, without a final U-bend, it might be allowing some pong in from there.