[Td5 headlight adjustment fault]

Hi Guys, trying to work out what's at fault before committing to purchasing parts. 

Both headlight adjustments are buzzing constantly. 

They change tone/direction when I turn the knob inside the cab. 

But they don't stop buzzing. 

I have disconneced the switch inside, and this doesn't stop them both buzzing. 

Could both wipac adjusters be faulty? Or is it something else like the switch.