If the sound stops when you pull the connection then its obviously not bearings or something spinning, have never heard of an alarm though.....
Do you have ESC fitted ??
Is it an alarm or a loud Hum/buzz ??? ...................... alarm, check breaksafe if its a hum/buzz then I would say brake magnet...
With van plugged into tug as per normal, pull the brake leads at the connectors at the hub, put a meter on them they should show no power if they show any volts then track back to 7/12 pin it could be shorting out, of course if ESC is fitted then it could be faulty.........
Where does it sound like its coming from when you are inside the van ? The BM or Setek unit isn't near the same spot?