I think it was intentional. It sounded like a buzzer that had gone weak from never being used. It would also occur at regular intervals, so I am sure it is an intentional warning. That would make sense since it is important for drivers to know that their instrument cluster has just died.
In another thread, someone discovered you can close/open your windows Volkswagen-style via the door locks and I think this is just another one of those little surprises that MB never bothered to tell anyone about.
To test it, W124 owners can simply pull the #5 fuse and listen for results. I am thinking it would also be nice to be able to disable this on demand -- to get out of speeding tickets:
"I am so sorry officer; my instrument panel fuse just blew and I am on my way to get a new one. See for yourself (holding up blown decoy fuse). " :p
(In my first post, I mentioned the speedometer worked, but I was wrong. Something else was still active but I am not sure what.)