Focus dash 'bing'

[TankRS]

wonder if anyone else has had this or knows what it is. every so often when i start my car theres a 'bing' noise from the dash. its not evertime it happens and has only been doing it on and off for the past few weeks. also notice that every now and then the current mileage readout resets itself to zero (top mileage readout not total mileage, if you get what i mean?)

my model is a MK1 Focus 1.6zetec 2000reg.

cheers for any help or info you may have.