Question about the PDC system

[AechX5]

Hi all. Thank you for accepting me into the club

I've just got hold of a E53 X5 and I've got a question if I can.

The parking sensors are playing up, I've heard this is quite common. I've checked each sensor and they are all making a clicking noise which I'm told they are supposed to do if they are working?

What's happening is, when I put the car in reverse the PDC will give out a long beep then the PDC button will just flash. Module malfunction? Or something else?

Also am I right in thinking that the PDC system is independent and will work without a head unit?

Again than you for accepting me into the club