This usually means the valve motor, the injector, or air being pulled in on the brine draw.

The draw stage pulls brine through a small injector under suction, so a restriction or a leak on that line turns into a gurgle or a whistle. A worn motor or a dry cam gives a grinding or clicking instead.

Note when the noise happens. During a cycle in the small hours usually points at the injector or the drain. All the time usually points at the motor or at plumbing water hammer near the unit.

Do not buy a new unit for a noise. Injectors and motors are parts, and the noise is usually telling somebody which part it is.