Well, after 20 hours of engine time and dealing with what I thought was a ground loop, I have isolated the problem area. I found it to be entering the intercom via the audio out on the 396. I was hearing a high pitched whine that followed the RPM of the engine. I went around the block on this one trying to eliminate the "ground loop" through what I thought was due to a double-grounding in either the 396 and or the PS 1000 intercom. Moved all ground wires several times trying to find the little gremlin. As it turned out, it was entering through the hot wire feeding the Garmin. I determined this after talking to a factory Tech. for about 15 minutes. He finally admitted that the 396 lacked in the arena of internal filtering. He said he rarely has a customer call with this problem when it is a 496 or newer. So, the fix was an in-line audio filter and now all is quiet as it should be. Just thought I'd share this one if it can help someone at a later date.