DIAGNOSIS That's almost certainly the condenser fan blade hitting its housing or a wire. The fan is behind the lower rear grill, and vibration from shipping or dust buildup can misalign it.
FIX Unplug the cooler. Remove the screws holding the lower rear metal grill/cover on. You'll see a small black fan. Gently spin the fan blade by hand and look for anything touching it—a stray wire, a clump of dust, or a warped plastic shroud. Reposition any wires and clear debris. Make sure the fan is seated squarely on its motor shaft.
IF THAT DIDN'T WORK The fan motor bearings might be worn out. If the noise persists after clearing obstructions and is a grinding sound, the fan motor itself needs replacement. It's a standard 12V DC fan, you can find a match by its size and connector.
HEADS UP When you put the rear cover back on, make sure none of its mounting tabs are bent inward where they could contact the fan. That's an easy mistake to make and will restart the noise immediately.