When your tow ball is fitted they it would be normal to adjust the electrics so the reverse parking sensors disable when the tow ball is in use (i.e. when there is something plugged into the lighting socket). This would prevent the parking sensors making a noise when a tow bar bike carrier (or trailer) in being used. Do check this to be sure your tow bar has been installed in this way. If not, you'll have to manually disable the parking sensors while you are towing, or they will pick up the tow bar bike rack.
Bike carriers