Water pump

It sounds like it is in the exterior compartment that houses the utility.

That's my guess being the hookups are in that compartment. How would you winterize it ?
There should be a pickup tube that runs off it and my guess is under the sofa near that compartment or in some cases, there is trap door setup inside that compartment. A door against the back side that can be opened to access the pump and valves attached to it. Sometimes it's hard to see because it blends in (same color) to the inside of the compartment on some models.

You should be able to hear it. It will be close to the fresh a water tank for sure, and that is usually close to the fresh water fill up point. That should help yuo narrow it doen.