I had a similar occurrence on my (now sold) 1991 300TE 4matic. I had a ticking noise, as you describe, from the front of the valve cover area. The car had an immaculate service history from the day it was new.
The noise was intermittent, one day it would tick, the next it wouldn't. One day it was ticking and I was near my mechanic, so he checked it out.
Turned out to be a failed/worn #2 lifter and a badly worn camshaft. #2 lobe was bad.
Some other top-end work was also done at that time.
Total cost to fix it all $1200. This car had its oil changed regularly, and while I owned it, it saw nothing but Mobil 1 10w30 at 5000 mile intervals.
Get thee to a good mechanic ASAP. I have my old camshaft sitting in the garage....