If the problem is thought to be the seat belt warning, could it be that the switch operated by the tongue of the seat belt is worn or the switch is getting lazy. NOT something to be done lightly but try carefully pushing an insulated screw driver into the seat belt stalk and see if the noise stops.
As to taking a seat belt off after being stopped by plod, in my day, a long time ago, it was what we saw NOT what we believed. Stopping a vehicle and then seeing the seat belt not in place should elicit the question when did you remove the seat belt. Courts would throw out anything that was the least bit ' well I am not sure that the driver was wearing a seat belt' indeed the CPS would not put it up for prosecution. They in my day would look for any reason not to prosecute unless it was a slam dunk.
As to seat belts and the wearing of, my approach to Ladies who I caught not wearing a seat belt was to draw my stick ( not always appreciated) and show them what a car windscreen did to a piece of African Iron wood. I had to break one at the scene of a suicide. I resisted attempts to have my stick replaced as it was a very useful illustration to Ladies what their face would look like after a trip through a screen.
