Could be about 20 different things but more then likely it's got a bad compressor or coolant leak if you found it in a ditch because those repairs are usually worth more then the fridge is worth. When you plug it in the compressor if it's noisy then gets quiet after a couple minutes that's a sign typically of the compressor going out but you'd need a pressure guage to make sure the return line from condensor to compressor is close to 0 psi. If you plug it in after sitting for awhile and the compressor doesn't make that much of a difference in noise after a couple minutes it may just be a coolant leak, inspect coolant line connections see if you have any oily substance at any of the connections if you do you may have detected the coolant leak. Your best bet is to call an appliance tech with the right tools for this determination.