I have a 2001 Mercedes S430 and this noise has been there since I bought the car back in March of 2015 (5K ago). Its a snow bird car in Florida, so it sat for periods of time as we also can see the marks on the cylinder wall in some of the pictures that I have from the borescope. Now the noise seems to be getting worse as the temperature drops into the low 60's as night. It sounds like a diesel engine running (at the peak of the noise) so I'm more worried now. This noise is very sensitive to temp and load of the engine but not the coolant temp. The noise starts after about 1 to 2 city blocks then it never really goes away but will get quieter as the temps of the piston and/or oil get hotter, coolant temp has no affect on it(see my theory below). It will get quieter mostly from a long drive and after a long drive it starts knocking in drive at a stop. Its always there at any rpm but 1600 to 1900 rpm in drive with light accel is the worst/loudest, the higher the revs/load the quieter it is in drive(load/temp at 1600 to 1900 rpms is what makes it knock louder not load/rpms) If you manual shift the trans you can really dial the noise in. If you rev it up in neutral as you are driving you can hear it but its for a second. (its not a rod bearing). This noise sounds just like all the cold piston slap/knocks that I have heard over my life time but this one never goes away. It could be that this noise is louder now because the outside temps have drop but it does not make any sense as temperature of the coolant as no affect on it, unless the oil is cooling the piston enough to cause this noise to be louder when temps outside are colder. This theory cross my mine as I know this engine has 8.5 qts of oil to heat up and these cylinders are oiled very well by looking at the internal design of this motor. This engine does NOT use any oil and the plugs after 5k look normal in all the cylinders, the engine has 99k on it now and I'm running Mobil 1 Formula M 5w-40 oil but when I got the car the previous owner had Mobil 1 5w30 oil. I don't know if Mobil 1 5w30 would cause any damage to the Alusil cylinders/pistons or not but we are not living in Europe to run on the Autobahn so 30 weight oil should be fine. Mobil 1 5w30 does not meet Mercedes requirements but 30 weight oil is on some of there oil requirements for this car. Some people have told me to just run it but as a technician it does worry me somewhat. I'm not knowledgeable on these Alusil cylinders so I have sent some pictures of cylinder 8, which is the worst that I found with a borescope to this thread to see if people can tell me if this is normal wear or not. If not what could of cause this scoring? Do these engines have a normal knocking sound in them or do other people out there have the same noise as these engines get miles on them?

Thanks for your time and input.

 

You sound like a generally-knowledgeable mechanic...

Not being able to hear the noises makes an internet guess pretty sketchy, but I will throw a couple of ideas/comments out for consideration and discussion.

Your comment about piston slap makes my first speculation center on a cracked piston. The noise will be different for different crack locations.

For something different - I have seen several vehicles with a cracked flex plate that would make similar noises. For the non-mechanics following along, the "flex plate" is the flat steel place that bolts to the end of the crankshaft and drives the torque converter. It is sometimes erroneously called the flywheel, but that is a slightly different design that does a similar job on a manual-transmission vehicle.

 

No they do not usually make noise . Mine has over 200k miles and is quiet

I would just leave it alone Till it fails some day
If it dies good used engines are under $500 just do a complete swap rather than open this one up. Engine failures in this car are very rare and that's why there are tons of cheap good used engines -it's just not a used part that has much demand