We have the same boat with the same refrigeration system. I found the thermostat for the refrigerator box spill over fan was connected to the freezer thermostat. If the freezer did not warm up the fan could not run and the fridge did not stay cold.
I rewired the fridge spill over fan to run off the 12V supply for the compressor. It now performs much better and runs when required.
If a previous owner of your boat wired it so the spillover fan had a separate 12 volt supply that fan could be the sound you are hearing.
Let us know what you discover, Bob