i had a ( big often talked about london) BMW dealer hard wire my Motorad II into my autocom sys.7 unit under my seat (they removed the plastic tray and ri-fitted a slightly different shaped one, sys.7 hard wired to battery) the audio / power cables run from under seat... under left hand plastic panel flanking fuel tank.... to the sat nav on handlebar mount.
i had the same excrutiating noise every time the sat nav "spoke" and a constant "humm" or "static" noise when the unit was not even "speaking"
the BMW fitters knew less than i did about autocom, the sat nav unit itself and were not quick at fixing the problem. change the sat nav "cradle"? change the audio lead? change the autocom unit itself? earthing problem? try it with a 9v battery instead of hard power? change the sat nav unit itself? they really did not know where to start....
it took 3 days (not 2.5 days wait and then an afternoon on the bench....it was booked in monday morning and returned to me thursday morning) - and apparently 2 hours of phone calls to autocom to remedy the buzz/humm/static noise.
it now works perfectly with phone, ipod, sat nav, 2 way (bike to bike) radio and rider-pillion.....
how did they fix it? well... instead of a normal "Y" split cable to allow insertion of satnav and ipod into the same mini jack port they have inserted a small red plastic box (autocom product) which inserts the 2 sources into the autocom unit...as far as i know there was no need for "screened" audio cables anywhere......
it was bloody boring waiting for them to fix it...if you're having a similar problem you could call BMW battersea and explain your problem - it was less than a month ago that they fixed mine......so should be fresh in their minds.