I was planning to run seafoam for about 100 miles before an oil change this weekend, but I suppose I should put new valve cover gaskets on and examine the timing chain while the motor oil is drained.

Yes, I believe there is a way to rotate the main crank while the valve cover is off to see the chain rotate. It'd be worth it to do for peace of mind, at the very least.

Guess I'll order valve cover gaskets as well, "and the bill grows"

I'm not a big fan of something like seafoam as I have heard that if you have gunk deposits it can break them loose and they get stuck places they shouldn't...
Also, do the additive/cleaning before you change the oil. Just my opinion.

I've been running the euro formula advanced full synthetic from mobil 1 for the last 40k or so, so there really shouldn't be large deposits anywhere.

Mostly because I've already purchased the seafoam, I'll use it prior to the oil change so I can get it flushed out of the system with the change. I might pick some of the other stuff up and do another oil change in november after the mystery oil treatment.

So for now my plan is to install this tensioner and give the chain a good checking over with the valve cover off over the weekend. I looked into the tensioner swap and it's apparently very straight forward, so no harm in doing it I suppose

I think I might have figured it out! I think it might be the harmonic balancer pulley. That would explain why the sound remained after belt removal.

I'll check that again tonight after work, this is the video that makes me think so:

https://www.youtube.com/watch?v=HvWwCEyWSx0

I think there is a big difference in the repetition rate of the noise. Your noise(s) seemed to repeat about once per second. The only thing that mechanically corresponds to once per second at idle is a complete revolution of the timing chain. Everything else is rotating faster than that. That balancer sound seemed different to me. I could be wrong of course. Good luck.

The chain and sprockets will exhibit advanced wear if the oil change history of the engine is poor.

Use a high quality oil and changed every 7500 miles from new? Chain and sprockets will look like new at 150k. Use cheapo oil and regularly forget to change it? They will be totally trashed at 150k. If you bought the car used and don't have the service history, condition could be anywhere in between.

Hello!

First post here and came across this thread while trying to research about M112 timing chain problems. My 2001 E320 Wagon has 242,000 miles and has had a rattling timing chain since around 210,000 miles.

The sound is EXACTLY the same as the video the OP posted and has the same rhythm at idle. All the timing chain components in the car are original and the car has been using Mobil 1 MB approved oil though I have recently switched to Liqui Moly 5w-40 with only a slight decrease in the volume of the rattle. As of now, the engine runs pretty close to perfect and gets 28 and above on the highway with a full trunk of heavy objects.

Right now I am contemplating whether this is something I can DIY or if I'll need to pay a couple grand to fix it. The tensioner replacement and the chain doesn't seem like an overwhelming procedure, given I can get it all done with just the removal of a valve cover. The guides on the other hand...

I've seen many threads that describe the problem that I've had, but towards the end, I find the resolution rarely gets reported which leaves me guessing as to what I should do.

Thanks,

Adam

My engine noise: https://www.youtube.com/watch?v=BJN7MUrmbRo

This is without the serpentine belt attached.