The metal tubes are thin and disimilar to the haed and will crack over time, not just platic cracks. We talking almost 2 decades of expansion and contraction of thin small tube on a think metal head which will not expand as fast or contract as fast.

The metal tubes were inspected by the dealer, all were ok. On that cylinder bank.

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1988 560SL
1992 500E
1995 E300D
2001 E55 AMG
2005 E 320 CDI

Years ago a customer's 380SE had a rap at idle. Everything under the valve cover looked good. I took off both heads, then rotated the engine looking for something. Found a piston not coming up as far as the others. Coolant had leaked into the cylinder from a bad intake manifold gasket and on start up the connecting rod bent. The rap was caused by the piston skirt lightly touching the bent rod.

Chas H, thanks. This noise got worse with rpm. You are the first person that has mentioned something this might be.

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1988 560SL
1992 500E
1995 E300D
2001 E55 AMG
2005 E 320 CDI

But what does that mean? They looked fine or they were actually working. Metal tubes are always going to look good. This is still the most likely cause of the noise.

Could be a bent rod also could be a rounded out lobe on the cam not opening the valve all the way or the timing brake on the head isnt working because of age or the anti knock sensor isnt working. We were just going for the most likely causes. If you have alot of power loss or fluid loss or smoke blowing out the back. But noise is usually due to lack of lubrication hence the tubes. You can take the head off and push out the piston to know for sure or measure from the crank side but the noise wasnt a knock to me it was a valve noise. Good luck in finding the issue.

The dealer unless you are gonna pay them to fix it doesnt give a rats behind what the problem on a almost classic car. Most those mechanics read from a manual these days and dont know as much as they should hands on wise. Again good luck.

Another Benz tech, where the engine is now, said its not the aluminum tubes. He said he found a weak lifter but thats it so far. The head is not off yet. I have to disagree about the dealer techs. My local small dealer has many very qualified and experienced techs. The shop foreman was the Benz territory rep for a while. These guys have seen alot. I wont know for a few weeks, as the tech who has the engine is going on vacation. Now I just need to get the E55 from the body shop. Driving the 95 e300d is going to kill me!

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1988 560SL
1992 500E
1995 E300D
2001 E55 AMG
2005 E 320 CDI

They are so good they couldnt figure it out for weeks you may want to take what they say with a grain of salt is all i am saying. If the lifter is weak that makes sense if its starved for oil, which only proves the point. Hydraulic type lifter, no oil, no hydraulic type action.