High volume alert bleep from radio/CD

  1. My model of MX-5 is: __nc2 prht
  2. I’m based near: __Lichfield
  3. I’m looking for technical help or recommendations on: __Bose radio/CD

I was driving along peacefully today with the radio off when the speakers produced a hi-lo alert sound which must have been at max volume. I’m assuming it might have been a phone connect/disconnect bleep. Really quite startlingly loud. Can anyone advise how to turn this down or off? Thanks.