It sounds like the Trailer Indcator "Tell Tell" ie it buzzes to let you know the indicators are working (Drawing Current). Sounds like either the unit is faulty or your towbar electrics are up the swanny

highly likey that it's the towbar electrics, they aren't the most well looked after aspect of the vehicle.
 
It makes noise when indicators are put on when electrics are connected to trailer so you remember you are towing mine done it was water in plug
 

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