sewer gas b/u from Kenmore Elite HE washer

I have replaced our old washer with a Kenmore Elite which is a high efficiency washer, the old washer was a standard washer. We never had any venting problems with the old washer. The new washer has been in for about 3 months when sewer gases began backing up when we do a load of wash. Has any else run into a similar issue? The new HE systems are supposed to use less water could that cause a venting problem? Any suggestions on a fix?

 

Many of the newer washing machines seem to have higher velocity pumps...maybe it is not leaving any water in the trap. You do have a trap, don't you? If you pull the hose out of the standpipe and look down with a flashlight, do you see any standing water at the trap?