I went to my FLAPS and asked them for cheap generic V8 motor mounts. These were basically rubber with a through bolt from a bottom plate. Those I welded via the bottom plate to some angle iron, bolted to the feet on my compressor (5 hp 60 gal Ingersoll-Rand) and bolted the angle via concrete anchors to the floor. I can't recall the price, but I would be surprised if they were cheaper than the $14 each ones from McMasters.
The other thing is my compressor is in my insulated, double sheet-rocked tool/security room. Noise is not bad, especially with the door closed.
I also understand if that you plumb the air intake so that it is remote that takes care of a lot of sound as well. I have not tried that.
Those isolator mounts previous posters referred to at McMaster-Carr look like they are made for this type of application--had I been armed with that information, I would have saved myself the time it took to fab up my contraption.
