Pete - if you look at the steering column, this will at some point protrude horizontally towards the front of the MH. On top of this could be a small box known as a annunciator. This is the one that gives off sound as you unlock the MH and the "ping" from your indicators etc. Maybe that it what gives off the sound you can hear???
Have a look at this thread.
If so, what causes it to "ping" is another matter.
Good luck