How to isolate a refrigerator?

Have you tried Silent Feet?

http://www.vibrationsolution.com/index.php

Depending on the age of the fridge, you may have some compressor mounts that need replaced and that could reduced the transmission of sound. Also try foam insulation in the ceiling below the fridge if accessable.
If not accessable try drilling a hole in the floor in the middle of the fridge footprint area and blow in insulation either foam or dry type. That patch for the hold won't be visible unless the fridge is out.

in terms of upgrades, if you can't fix the problem with isolation (like feet or cork), buying a quieter fridge is pretty cheap - you might even find one for free on Craigslist. I have a 7 or 8 y.o. kenmore that's extremely quiet - the neighbors gave it away when they remodeled their kitchen - and it's much quieter than my previous one.

I did a search on the most quiet Fridge around and this name continually pops up every time. Might be the best upgrade you ever made...

Liebherr is the worldwide leader in premium refrigeration.

With over 50 years of experience in cooling, the German manufacturer sets the pace with continuous product innovation and a proactive approach to responsible manufacturing.

Liebherr brings fresh, European design to the North American marketplace, providing outstanding built-in, freestanding, and fully integrated refrigeration and an extensive selection of wine storage units.

it's true that you can spend megabucks on super-quiet (always European) fridges, but recent vintage, energy-efficient domestic ones can be quiet enough (+ cheap) to fix your problem.