2000 BMW 328 Parking sensors

Hello. My e46 328 coupe has non working parking sensors. Any advice welcome as I think I have tried everything! When I select reverse I get the 4second gong to say there is an issue with the system. My wing mirror dips and reversing lights come on so that switch is fine. All 4 sensors are ticking so I would assume that means they are getting power and working. The fuse is fine. I thought it might be the gong itself as on the obc gong test it is silent but after replacing that still nothing. Any ideas greatly appreciated, thanks in advance.