Originally Posted by A. Rosich

Hhmmm....

My guess is that you have confussed the rear electric pop-up masts, located at each rear corner on early W140s with the "Parktronic" system, option installed on W140s from 1996 onward.

The Parktronic system emmits a warning tone and shows how far an object is from the car with a series of warning lights, all of these done from a series of radar sensors located on both, front and rear bumpers.

The rear pop-up masts which raise when reverse is engaged do not emmit or send any sound, they are just a visual aid to the driver to help locate the rear corners of the car when backing up.