How do I stop interference on my Nissan Almera radio?

I have a 2003 Nissan Almera 1.5s. The radio works fine in the day but as soon as you put the lights on there is a loud buzzing noise through the speakers that drowns out all the audio. It only happens on MW and not on every station. The problem is I listen to BBC 5live alot and it is particularly bad on that station. It's not just the exterior headlights either. During the day, if you open the car door whilst stationary, the interior light comes on and the noise starts then for a couple of seconds. Please help as the dark nights make this more of a problem in the winter.

Thanks in advance

Golan
October 2007