Fridge Noises

Noticed this just before my parents left: The refrigerator in the kitchen is making noises.  Not all the time, but once in a while I would hear this hard hum, and then it would stop, and then it would start again.  There will be a series of these hard hums, and then it'd stop.  Never heard such things before.

Do the compressor coils need cleaning?  Is it the water filter?  Is it something worse?  Because it's probably something worse, knowing that I want to take a vacation next month.  Dammit.