I'm nearly at my wits' end with this after working on it for the last month. It's a 2012 GL350 BlueTEC with the OM642. It started rather suddenly, going from a faint do-I-hear-something while coasting down a hill (engine braking, really - it was in gear) to holy-heck-shut-it-down-now in about three days and 20 miles. Here's the sound:

https://www.youtube.com/watch?v=kAOI3RRWVUo

I've timed the ticks and they're at about half-speed to engine RPM. When the engine is turning over, there is no noise but as soon as it turns over it makes this noise. I never had any cold-start clatter that would indicate a timing chain and the videos of timing chain noise that I've seen all have a much higher speed tick.

So far I have:

• Disconnected each injector, one at a time
• Removed the accessory belt
• Replaced the chain tensioner
• Put 6 new lifters in the left bank (because I really really really thought I heard a noise coming from the left bank exhaust valves)
• Checked the valve springs for damage
• Removed the heat shields and looked for soot and used a stethoscope and a hose to listen for exhaust leaks
• Performed the timing chain test in WIS, getting only 8-9 degrees of deflection (WIS indicates 11 degrees or fewer is acceptable)

It's really hard to tell where it is the loudest - I have probed the whole motor with my mechanic's stethoscope. The injection pump seems quiet but the upper coolant elbow seems to be resonating the tick very loudly. It seems loud over the cylinder 1 valves on the right head cover, but it's also pretty loud on the turbo and the exhaust pipe from the turbo to the DPF.

I am pretty much out of ideas at this point. The only diagnostic step that I can think to take at this point would be to replace all the valve springs, keepers, rockers, and lifters since it seems to be ticking in time with the valvetrain. But, with due deference to our gracious hosts, that's a pretty expensive shell to load into the parts cannon when I don't even know if it is actually in the valvetrain for sure.

Any ideas? Hints? Suggestions? Cockamamie theories?