2004 Micra XS Light buzz on dipped beam

My daughter has a 2004 Micra XS, which has been recently serviced by the local Nissan dealer. For some reason a buzzing sound has now started when the headlights are turned on. It is not the "ignition off, lights on" warning. I have traced this buzzing, and it is coming from the off-side (driver's side) headlamp holder assembly. If I move the headlight, it varies in pitch. If I turn the headlights off, the buzz disappears. If I switch on main beam, the buzz disappears, otherwise it is there all the time whether or not the engine is running. Can anyone shed any light on this one (if you'll excuse the pun!) Thanks in advance.