How to isolate a refrigerator?

Why don't you just have the condencing unit moved to a new location?
All they have to do is:
1. Pump the system down, close off the service ports and cut the lines.
1a. All freon is pumped into condencer so you do not lose any.
2. Extend the refigeration lines.
3. Move the electrical disconnect or extend the whip from the electrical disconnect if the unit is not moved too far.
3a. Extend thermostat wire.
4. Evacuate the lines to air handler.
5. Open condencers lines.
6. Add freon maybe, could need a half pound if it is moved further than a few feet or if you were low.
I know this sounds like a lot of work but it can be done in about 4 hours, easy.

You could also have a tech come out with the types of pads that WE use to see if it helps. We use differant types and amounts of isolators depending on the load.

This could also be the line set from the condencer to the air handler vibrating. They may not be supported correctly. If that is the case, isolators will not help. Try having someone hold the lines outside, lightly pulling up and or pushing down to see if that minimizes or changes the noise, I do mean lightly. you do not want to crimp the lines.
Email me so we may talk if you are interested.
I do this work for a living. hvac/e = hevac1

The distance between A/C units in a split system will not affect the efficiency of the unit unless the length is over 100 feet. Copper line sizes are compensated for distance. Split A/C systems are designed and factory charged for up to 50' of copper lines between units. When the copper line sizes are increased for a run over a 50' Freon would need to be added. If the line size was not increased for a distance over 50' then efficiency would suffer.

Ductless splits are the exception, 35 feet max, anything over that distance would affect efficiency.