rough running 1987 560SL

I have just purchased a 1987 560SL with 94k original miles on it.  It 
clearly has an engine problem & I would like some advice on how to 
diagnose it.

The car, apparently, was running fine until one day, the previous owner 
started it up & it had a tapping sound & was running very roughly.

This problem has nothing to do with the timing chain, rails, etc. as 
they were just recently replaced (about 2 years & 10k miles ago)

Could this be a camshaft or rocker problem or ?

Thanks in advance for any suggestions on how to proceed with this.

Brian