The ususal suspect on the 606 is carbon. The valve springs are pretty light
for those valves, and when you r&r the intake or plumbing, carbon can tumble down and get stuck on the valve seat. This causes a heafty compression leak and misfire.
The compression pops are what you are hearing..."knocking."

The non turbo 606's are cool, because you can sit there and watch them run with no intake manifold.
If you do have a leaking intake valve, finding it with the intake off will be easy, rev her up and watch for sparks getting past the valve. That will be your carboned up valve.

Beware of the busted chunks of plastic fuel line retaining clips. They love to fall into open ports when pulling the intake. That will be a BIG knock that does not go away when rev'ed up.

Regards,
Dr.Diesel

__________________
Dr.Diesel -
Shop Foreman
Master Guild Mercedes-Benz Technician.

Think the dealer is expensive? Do you know how much it costs to train a dealer tech? Or buy the required MB special tools to have on hand?