High pitched whine?

Our 3 year old Admiral side-by-side USA-style fridge freezer is making a high pitched whine. This varies in pitch from time to time, and sometimes reverts to its normal, fairly loud, hum, but later comes back. It is a very irritating sound, but more than that, it leads me to believe that the machine may be losing some coolant or has a faulty pump or compressor, even though the machine is still working well. Can anyone diagnose this problem or offer a cure?