I have an '97 S500 that has a intermitent ticking sound coming from what sounds like the top of the motor, passenger side, directly under the valve cover. I spoke with a highly qualified SAE mechanic whom gave me full access to his on-line diognostic programs. It took some time, but I dug up a little known fix for this common problem in this range of years, in cars around 100k. The problem I was told to look for, and found, was a leaking "oil bridge". This is a small tubular part which sits on the top of the head and assists in oil delivery from the intake side of the valve train to the exhaust side. There are 8 of these per head. They will crack with age, leak and essentially starve the exhaust side of the valve train of oil. If they only crack, your car will tick on start up, your lifter and rocker not getting enough oil until everything heats up and the expansion of the plastic shuts the crack. These parts can get bad enough that they don't self heal with heat and your tick will not go away as long as the engine is running. This is a simple, backyard/weekend fix by merely removing the valve covers and replacing the oil bridges (I'd suggest doing all 8 at a time). Suggestion would be to replace your valve cover gaskets as well.