Sounds like classic worn 2.0/1.8 dephaser pulley. Relativley quite from cold and once the engine gets warm the dephaser locking pin disengages and the rattling / diesel sound starts and you can't place exactly where the noise is coming from as it's across the length of the cam shafts / upper cylinder head.
As already stated if it was worn hydraulic lifters they would make a noise right from starting the engine from cold.
It's not always easy to detect movement in a worn dephaser as when the engin is stationary the locking pin deploys so you can't move the dephaser.
I have had 2 noisy dephaser pulleys apart and they showed virtually no signs of wear. I made some pins up and locked the mechanism's and the engines became permanently nice and quite.
I have recently bought a 1.8 F4P from a breakers. It uses the same dephaser as the 2.0 F4R. It was supposed to be a good runner! I got the engine in the car and it sounded absolutely awful. It sounded worse than a London bus. When I took the dephaser apart several of the chambers where broken as where all but 2 of the veins which wasn***8217;t allowing the dephaser to return to
. Also the dephaser solenoid was jammed up with a shard of metal that had found it***8217;s way through the
ways. This dephaser I had to weld together. The engine is nice and quite and the car goes like sh1t off a stick you would think it***8217;s a 2.0 not a 1.8
I didn't have to repace the hydraulic lifters in any of the 3 engines. There was some scoring on some of the cam shaft bearings .