Thanks for the replies so far - it's very much appreciated! I will try out some of the suggestions when I get home from work. The FDM DUO was purchased for use in the shack where a Yaesu FT-DX3000 and an Icom IC-7400 perform flawlessly - on the same antenna!
The shack has a fantastic earth system which is just 3 feet away from the rigs. The radio sounds like a noisy old AM transistor set from the sixties, LOL. It's definitely picking up every last bit of noise in the room/area, where the other radios are lovely and quiet. Again, inside the car (fed from a LifePO battery) the DUO is terribly noisy whereas the other radios are just not affected.
Very odd!! Anyway, I'll try out your suggestions and get back to you tonight.
THANKS AGAIN for your support.
Tom - M7MCQ