If it makes the noise while you're sitting still then that's not the diffs. That little tid-bit of info would indicate something engine or accessory (Fan, AC compressor, alternator, PS pump etc.) related as those are the only things in operation while your stationary and running. In that case you need to pop the hood while the engine is running and just alternate from one fender to the other while you gaze into the wonders of the engine bay. Hopefully your ears will point your eyes to the culprit.

If you look in my gallery I've got some pics of the drain and fill points for the diffs. Locating the fill and drain is the hardest part of doing the diffs.

As for your cruise control, there really isn't any additional strain put on the engine. The cruise control actuator is simply a servo that moves the throttle based on your speed preset and info from the speed sensor. Since it doesn't move the throttle as aggressively as your foot it can take some time to reach the kick-down point for the tranny to go into a lower gear for inclines, and that can get annoying in hilly areas.

Keep us informed on the noise issue, I'm curious with it happening while stopped.