problems with wet/dry filter - eheim 2227

[gator]

I have an eheim 2227 wet dry canister filter that is about 2-1/2 months old.   The way it works is that it fills up to a certain point,  a floater activates a switch, the pump empties the canister to a cerain point, and the cycle begins again.  Last saturday, the filter started making a  sucking noise once all the water was pumped out, like it didn't know to let the canister fill up again.  I've tried everything in the manual, taken it apart and cleaned it, examined all the seals, but I can't seem to get it to work as it did before.   Does anyone have any simialr experience with this filter?  (the Big Als in Oakville has offered to replace it free of charge, but if its a simple fix, I don't want to waste their time/money)

Thanks!