Try connecting a spare coax (radio shack) to the transponder and antenna and routing it away from the other cables and wires. If things quiet down then you are on the right track.

The strobe light noise in the audio also is a big clue. Although Lightspeed Zulus are known to eliminate transponder noises, not pick them up, you might check anyway just to see if they are picking up any extra noises. Using just one headset, move about the cabin to see if the location of the headset or wires changes the noise level. If the audio cables are picking up noise through the wire bundles you might have to move them as well.

JP