[Rclark]

 Was in Radio Shack and bought an isolator just to try it because my DAC has a mechanical hum at times noticeable when everything is off.

 I plugged this thing inline between the dac and my amp and bzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz it didn't work at all. Actually added a very loud hum/buzz. Seemed to have an effect on the audio but didn't leave it in long enough to guage.

 Am I using it wrong or do I need to return it?

[Speedskater]

An analog audio signal isolator won't have any effect on a mechanical problem.
Try again using plain-jane analog audio co-ax interconnects.

[Æ]

Take a look at this schematic, drawing. It shows a circuit for removing/blocking DC that might be polluting your AC power and getting into the equipment power supply. Apparently at times DC can ride along on the AC mains and cause transformers to vibrate etc. If the pic is too small to see, print it out.

[Speedskater]

Take the DAC to a friends house in a different neighborhood and see if you get this mechanical hum.

Many "DC" offset problems are intermittent depending on if the offending device causing the problem is on or off.

[srb]

If the hum isn't due to DC offset and the transformer is a toroid, sometimes loosening the center bolt and rotating the transformer a few degrees and retightening can help.
 
Steve

[Rclark]

Eh, I returned it. But I will try that last one.