That's a new one for me I'm afraid, can you isolate the issue? You could go through some basic fault finding if you have time - Does it happen only on dipped beam, does it do it when all lights off and you flash main beam, with/without side lights, DRL etc, try all the combos. If it's only only one type of light can you remove one of the bulbs to see if it's a particular fitting? Might help narrow it down ...
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It happens with the side lights on, then gets louder with the main headlights. Not checked with all lights off then flashing full beam on yet but I can test that tomorrow. If I disconnect the rear speakers directly front the head unit the noise is quieter, but its still there. I'll certainly try what you suggested thank you.