Why is there an occasional rattling sound from the rear?

Good afternoon Bob.

Wine coolers have a lot in common with mini-refrigerators. The cooling for the wine, to proper 'cellar' temperatures is done gently (one hopes) with circulating cool air that should keep the temperature stable, and the dust to a minimum.

The fan you hear is most likely one keeping the refrigerant compressor cooled off. Depending on how your unit is packaged, what you'll likely want to do is

1. Unplug the unit. No pokey pokey into the guts of things plugged into the wall. Always err on the side of safe. Aside from shock rick, the small fans in fridges can actually cut fingers up pretty good if you let em.
2. Look to the rear lower section of the unit and find the compressor(s) and associated fans and perhaps cooling coils. Much like with regular fridges, use a brush or a vacuum attachment to clear any dust bunnies or other debris off of the coils, compressors, and fans.
3. Examine the fans. See if anything may have been sucked up into the blades or spindle - giant cobwebs, bits of thread of carpet fiber. See if the blade will spin freely, and whether it feels loose. If it won't move, or is obstructed, try to clear it and clean off any debris or fiber. If it will not move, the fan may have failed. You don't want to run a fridge this way, as the next thing to go after the cooling fan for the compressor is the compressor. At which point most honest techs will say 'buy a new fridge, I can't replace it for less than the fridge will cost you.
4. You can also see if there is anything else obviously loose near the compressor or fan. There's a significant power surge needed to start the compressor, and the initial slow cycling may be making things shudder.

Fan blade is striking something. Needs to be bent back into correct position.