How to isolate a refrigerator?

Electrically I would suggest putting an Audio Prism Quiet Line filter on the fridge outlet. That does help keep the grunge from the fridge off the house line.

As for mechanical isolation I would suggest looking at the same things that you use in an audio system ... sorbothane pucks and such under the culprit, additional bracing in the floorboards and support struts, etc.

PTM,
The noise is strictly mechanical in nature and is coming from the AC unit, located outside of the house, close to the basement wall, where my speakers are

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

Hevac,
Thanks a lot, very helpful. Im actually having a HVAC guy coming over to look at it. Will see what options are available.

Moving the A/C unit could solve the problem but the longer the lines the less efficient the A/C is. But a small trade off for the solution you seek. If you do have it mived have a extra thick concrete platform poured. That will help in vibration. One thing to answer your question is, yes the a/c unit needs to be level, and isolating the tranfer lines from the wall pass thru will help too.