I think I first noticed this last year but had forgotten about it.

If I switch my ignition on (not starting the car) I can hear an irregular light knocking sound from the dash area of the car. I can hear it if I open the luggage compartment too, but it seems slightly louder in the car. It is irregular, but frequent - i.e. a few times a second.

If I turn the headlights on (even the sidelights) it immediately stops. If I turn them off, it immediately starts again.

I can't hear it with the engine running, but that might be because it's drowned out.

Any ideas? If it happened with the lights on and not off, I'd assume some kind of headlight levelling issue - but this seems the wrong way around!