Problem: When the intercom is powered up, the CRS speakers emit a motorboating-type of noise. The speakers are cabled via the patchbay. The noise is the loudest when the console is off. I made a recording if you want to hear it. Also a few intercom lines contribute the most to the noise (17, 18, and 20). When they are unplugged the noise diminishes significantly.
Solution: First I wanted to test the lines from the intercom to each respecting digital keystation. This is a 9-pin D-sub connector on one end of the cable and a RJ45 on the other. That wasn’t an easy test with the equipment that I have so I decided to go looking in a different direction. I wondered about the patchbay grounding. The intercom doesn’t go through the bay but the speakers do. Plus most of the intercom cable runs near the bays. Each bay had a ground hook-up wire attached and each hook-up wire terminated to the other bays by spade lug on a equipment rack rail. However, the rail was not grounded. I decided to ground the rail back to the console. This is done with a 14 guage hook-up wire to a screw on the console (see picture). Ta-da. No more noise.
[Edit. Did a live sound event yesterday (8.2.09) and I still have noise from the digital intercom system at the live sound console. Well, nothing is easy. I'll update the post when I find a solution.]