Chain stretch isn't really a thing on a well maintained double row chain. The tensioners really don't go bad as well if you maintain oil changes regularly. They go bad when you neglect oil changes and the oil pressure galleries get clogged...

The tractor sound is injector nailing. Bad delivery valve springs, leaking delivery valves, injectors that aren't broken in yet, worn or leaking injectors will cause nailing as well with other variables too.

It's Nothing to worry about on a benz diesel, and it's pretty normal. Sometimes it's easy to rectify and sometimes it's not. If it's still getting good MPG and good power and not smoking a lot, it's fine.

I'm at 210k on original injectors with very little black smoke at WOT so I'm not bothering to send mine to greaazer yet

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Only diesels in this driveway.
2005 E320 CDI 243k Black/Black
2008 Chevy 3500HD Duramax 340k
2004 Chevy 2500HD Duramax 220k