This may not be relevant, but I had a very similar fault recently on my old Nissan Bluebird. When you switched sidelights on, the lighting control unit (behind the radio) buzzed. When you switched headlights on, they worked dip and hi-beam, but the 'beam' light was permanently on.
Turned out just to be a blown bulb, but blown in such a way it was shorting something out; the bulb had partly welded itself to the connecting plug and had to be extracted using a pair of pliers. So first, check the bulbs before changing the lighting control box, which is expensive (90 pounds for the Bluebird; fortunately I didn't need one)