It may need a new piston, seals and spacer kit. That isn't easy for a DIYer to do but it can be done.
Or it could be the meter turbine on a mechanical metered version or the 'boot' in a brass or SS bypass valve.
You may want to have a local dealer come outto look at it and fix it. Do not beleive any that say it can't be fixed.
There is no such thing as a "salt free" water softener they do not remove hardness and are not softeners.