If they are parking sensors then you should be able to see them in the bumper and preumably they only operate when you are close to anything you might hit.
If this warning buzzer operates whenever reverse is selected then it may be a revrse alarm
As service is free for Motability then presumably the dealer should be able to find any history of the such an item being dealer (or factory OE)fitted because you can bet it's not a motability driver fitted mod
I think that theloud noise you hear when moving in reverse is to make the people behind you aware of what are you doing.
there are 2 posibilities:
-you have an alarm that offers that feature. in this case, you should check the manual for that alarm, because usually those features can be disabled using the alarm's remote and the "valet button" of the alarm
-you have light bulbs for the reverse lights that incorporate a buzzer. in this case, just change the light bulbs with normal ones.