As I said before, can you remove the harness from the car and listen to the front/rear pairs (not the home outputs) when powered from the home adapter. If the noise is still present on the 4v outputs, I'm suspicious of a production fault - it sounds to me like the 10v PSU jumper hasn't been fitted, which means the chip that powers the 4v outputs is being powered by the inputs. Erk.

This shouldn't be able to get through the automated testing, but it's a different factory producing the mk2's and I can believe that one person on the line who isn't quite up to speed could make such an error.

Hugo