Started up the car this morning and the radio came on as usual (I
usually have it defaulting to 'on'); however, the normal music and chat
was accompanied by a high-pitched 'beep' about once very second. I've
switched on and off, disabled all the radio's clever functions, but
can't get rid of the beeping. What on earth is it? Help please!

Background:
This is an original factory-fitted radio in a 1998 Renault Laguna (think
it's a Blaupunkt?). I've had the car 5-6 years so am very familiar with
the radio; the noise isn't related to traffic alerts or anything like
that. Happens on all channels, on both FM and MW.

The radio has a security feature whereby you need to enter a PIN if the
power is interrupted; however (a) it wasn't interrupted; (b) it goes
into a mute status with a PIN prompt on the display; mine didn't do
either. I just mention this in case anyone thinks that's what the
problem is.

Any suggestions would be gratefully received!

Thanks
David