Thanks for all the help. I solved the problem two days ago. One of the exhaust manifold gasket(s) needed to be replaced. The problem get much worse and eventually I could tell with no doubt the cause behind the ticking noise. I changed the gaskets on the driver side and the noise is gone.
You guys were right on the money. It is disappointing to know that a Mercedes shop couldn't tell the problem while you guys ,without even seeing the car, could provide helpful hints
That brings me to a new question. The ride is my car is not that great when compared to my brother's 78 450 SE. I have been told that the ride in the 560 SEL (Due to the rear self leveling) is not that great. Is that true?
I have changed the air cells, filter, and Hydraulic fluid with little change in the quality of the ride. The problem is worst while driving in town. On the highway the car rides great. I also noticed that the passenger rear tire in not even with road while the driver side rear wheel is perfectly aligned. Any ideas?