An Update !
Last week, the problem got a lot worse.
The car wouldn't accelerate when cold, and the diesel-like sound was even worse, than before.
After a carwash it got even worse than that...
It turned out, that the water, dampnes..., inflicted a problem with the sparkplug wiring.
When I started to wipe the sparkplug leads dry, I found some fresh oil in sparkplugwells on cylinder 1 (closest to the oilfilling cap) and I found some black, dirty and sulfur smelling black oil in the sparkplug wells, at cylinder 4 (closest to the battery).
Cleaning everything and drying it.
Wow, its a completely new engine !!!
Sound silk-smooth, and the fuelmilage is a looooot better.
The fresh oil, and the water is on my account, and only made the problem worse.
The smelly black oil, comes from a very tiny little crack in the gasket.
Takes about 500 km - 700 km (a week) before enough oil has leaked, that the problem with the diesel-sound, begins again.
Fixing the gasket, fixed the problem for good
Thanks to all, for you'r advises, that made me wiser on the TS engine.