Did you check the belt tensioner shock absorber? It's known to make a clicking sound when worn out. With the engine idling, put your finger on the shock absorber and see if the noise goes away. It might also be a good idea to check the timing chain for stretch. I recall reading that some of the OM602's had bad timing chains. Using a quality synthetic oil is always a good idea regardless of the engine noise.

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2004 VW Jetta TDI (manual)

Past MB's: '96 E300D, '83 240D, '82 300D, '87 300D, '87 420SEL