Check the water level in your brine tank. If the tank is not refilling properly you will be sucking air in slow rinse if the water level is below minimum for the cycle length.
Check the full function of your softener control head. Depending on age this may be some bad seals inside the head.
It is not uncommon for the city water to have air in the water and this would have a chance to form at the top of the treatment tank. Do your neighbors have this problem too?
David