Parking Sensor sounds for 10 seconds then stops

2003 TD5 HSE Disco 2 - the parking sensors sound (constantly not beeping) when you engage reverse, as if you are already up against something. The switch on the dash is flashing. Then the sound stops after 10 seconds and it doesn't alert you to anything. I have read several posts where the system fails and no sound, plus flashing light, but this is not the case here. Any ideas what to check first and can I check with multi meter? Thanks for your suggestions! - Paul