Hi,
I would expect it to be an ABS sensor. I had a similar problme in a MRK3 golf GTI. Baiscally it buzzedd when you applied the brakes, it transpired to be one of the ABS sensors, which when replaced sorted it out. It may be that the sensor ring is$ blocked or damaged, Possibly with mud.
If the sensor is gone then either a dealer diags kit, or multi meter should detect which one.