Is it the compressor? You can try to cover the compressor with something to muffle the sound, but be careful because you dont want to cover the radiator. You dont want to overheat your compressor. Also, try putting some clothe sticky-things that go on the bottom of chairs to keep them from scrapping wood floors. That might help with sound transmission to the floor. Other than that, try tightening the screws that hold the comressor to the fridge.
-Jefe-