The Power supply to the connector for the Sat Nav ( on the frame below the left mirror )
is a "clean" Power supply that should not pick up any interference.
I'm not familiar with the Zumo 350 - Does it have an audio out cable to connect to the Trophy NAV Audio ?
One way of checking your set up would be to substitute any cables you have used
with other cables that are known to be "good" - in conjunction with trying your cables
on another Trophy to see if the interference is replicated on the other bike.
