Your refrigerator, though not installed in the slide, is mounted "kitty-cornered" and has two side vents (no roof vent). On that type of installation, there is not enough "chimney effect ventilation" to keep the rear coils/refrigeration unit cool, so a 12 volt fan (very similar to a computer waffle fan) is installed in the space between the vents to blow air across the refrigerator coils. Quite often the fan doesn't make enough noise to be noticed until the fan blades get dirty, then the unbalanced condition causes it to vibrate. That's when it often gets loud enough to be very noticeable. Dometic doesn't consider the noise to be a warranty failure, and cleaning the fan will sometimes solve the problem.
The fan is connected to the red wire that comes from the black plastic control box (usually on the rear lower left hand corner, behind the vent cover) and has a fuse holder in it. On many Dometic refrigerators, the fan is thermostatically controlled and doesn't run all the time, but on some models, it's a 24/7 fan. You can disable it by removing the fuse in the red wire if the temperature is not going to be above about 85-90 F. Any warmer than that, you'll need the extra ventilation to keep the refrigerator operating efficiently.
I'll try to find a picture of the wire/fuse holder where it exits the control panel cover.
ADDED: The photos of the back of the refrigerator are located in post #10 in the thread found here:
http://www.keystonerv.org/forums/sho...t=refrigerator