ECOWATER ERR3500R20

We recently moved into our new home. There is a water softener installed (see above for model number). Whenever we turn on the water in the house, there is a loud rumbling sound coming from softener. I checked the salt level and re-set the unit. Since I am my familiar with water softener, I would like to know if this sound is normal? I tried calling the company in San Diego,but I get the answering machine and never get a call back.