Escort Mk4. Just started doing this and it's constant!

Whenever either door is open, the central locking is repeatedly switching from locked to unlocked to locked etc. Whine-click-whine-click-whine-click...

When both doors are shut it stops and they are then permanently locked. I pull the inner handle to get out and it opens fine but whine-click-whine-click... til i shut it. This also means that when getting in from the outside I have to turn the key in the lock and open it quick while it's momentarity in its unlocked state

What's happened and how do I sort it?

Thanks, Chris