My Zanussi ZJ1218 has a bad smell even after a service wash, within days the smell is back.The waste pipe is looped to top of machine before connecting to the sink waste pipe just above the u-bend. Have cleaned out u-bend, the same smell was even stronger on removal of the trap. Also always cleaning grey slime from door seal and pump filter. Can the water from the u-bend siphon back into the machine?. PLEASE HELP.