Here are the possibilities that causes excessive oil consumption. these are take directly out of my mechanics mercedes training book...
1. external oil loss (sump oil pan)
2. Ruptured diaphramn in vacuum pump
3. Damaged valve stem seals.
4. Valve guides worn beyond specifications
5. Excessive cylinder and/or piston ring wear.

As for the clattering on your 240D....could possibly be your injectors...when did you last change them out? I know that if your injectors are leaking or bad, then the glow plugs might get destroyed which could lead to a long delay on the pre-glow system. Here are the possibilities for excessive engine knocking
1. Air, water or dirt in fuel system
2. Inferior fuel quality
3. Pressure valve holder over torqued (renew copper seals rings
4. Injection nozzles fouled or opening pressure incorrect.
5. Depressed connector of injection lines (Reduced inside diameter
6. Injection pump timing incorrect
7. Inadequate compression
8. Broken pressure valve.
9. Valves seated too deep in cylinder head
10. Injector nozzles over torqued in head

thats about it...if you solve your engine knocking....chances are you'll solve your smoking problem. Also check your timing chain and tensioner...When was the last time you changed these things? The chain is supposes to last 60,000 miles. If the chain is advanced, you'll get heavy smoking emissions at idle. I just replaced my timing chain and readjusted the valves for the first time....my 300D 1978 work like a charm and it has 285,500 miles on it. My dad is the first owner too

Good luck and i hope this info helps you out