Commercial ground loop isolators from Radio Shack and Vetco Electronics have let me down. They have about 700pF of capacitance between input and output terminals. If the ground network is carrying high frequency signals like 10kHz this capacitance lets a lot of that signal through. So I decided to build my own using 600 ohm 1:1 audio transformers. They demonstrate an I/O capacitance of about 46pF, but I'm not sure how accurate my meter is in this range (+/- 20pF is my best guess). Here's a pretty pic of the finished project before the cover goes on top:

The transformers were $2.19 each, the 3.5mm TRS minijacks were $2.99 each. The case was $12.99. A little hot glue holds the transformers to the case.