I have a Kinetico K-30 water softener that is about 15 years old, and lately it makes a loud humm when water is running in the house. You can feel the head of the unit vibrating when it hums, like there is a loose washer or something. Is there regular maintenance that should be done on this unit? Any idea of what it could be? Also, I have the original resin beads - is it about time that the resin should be changed as well? The unit has been great - no problems up until now. Am on city water. Thanks much .... -dog