rattle from water pump

if the rattle was itemised on the work detail at the last service Mercedes would have made a written comment other than Not Concerned... rattles from the water pump area could be from worn timing chain tensioners.. try running the engine (briefly) with the waterpump belt slackened or removed this will prove the pump.. and the tensioners. there is not much in the waterpump to rattle unless the bearings are bad and then you would be able to rock the shaft and the pump would probably leak anyway.....

I can almost guarantee you that the "rattle" is nothing to do with the waterpump at all. It is a very common occurrence on the Mercedes engines with the poly-v belt. The "rattle" is actually the belt flapping about - you should be able to see it if you look down from below the powersteering pump along the belt with the engine idling. The reason is a stretched belt and/or worn belt tension damper unit(looks like a little shcok absorber).
I'd say get the belt changed ASAP. It is not very hard at all. You just need two 13mm spanners and the back(straight) end of your standard wheelspanner.
I'll try to describe the process here for you, though far easier when you have it in front of you...This is how it goes on the diesels; the petrol ones are very similar from memory:
Start by undoing the 13mm nut just above the belt tensioning spring. Then insert the backend of the wheelspanner into that unit which the spring is attached to and pull against the tension of the spring so much so that you are able to push out the bolt of which you just undid the nut. Once the bolt is out of the way, you can slowly release the tension of the spring. Before removing the belt, have a good look at its orientation, as it can baffle some people. Anyway, so now you can remove the belt. Once off, you can check the water pump, power steering pump, alternator and idler pulley for smooth running. Also, check the damper - it should have firm resistance against movement.
Fitting the new belt involves getting underneath. Form the one end into an eye approx 3 inches diameter with the grooves on the outside. Slide this over the idler pulley and then proceed routing the rest of the belt over the crank pulley, water pump, alternator, and finally the powersteering pump pulley. Once on, just pull back against the tensioning spring with the wheelspanner, re-insert the bolt, fit and tighten the nut and you're done. Should take no more than 5 to 10 minutes :D
Now start engine and check/listen. If the "rattle" is still there, it is most certainly the damper. I always replace the belt first on its own as that is half the price of the damper. Last time I bought one from Main Dealer, it was £45 plus vat(diesel engine version).
These belts should be changed every 20k and get often overlooked. I tend to change them every 10k as they break without warning. And when they do, it turns out a real mess with long thin strands all wound tightly inbetween the waterpump pulley and viscous clutch. Takes ages to get them out :x
Good hint, place spare belt under seat or somewhere as a get-home. Add long 13mm ringspanner to toolkit.

Hi Can fax you 10 pages from the 463 service manual with all the info about belt. installation. etc.

the book says " power steering 1 ltr" of steering box oil.... if oil smells burnt or is very discoloured it is best to drain and replace....

Hi rule of thumb says it should be about (in old money) an inch from the top of the reservoir with the engine Not running.....

I recall that it should just cover the flat white disc in the reservoir which is only half full. I can't seem to find the paperwork
Is that what you are describing?

There is actually a top and lower level indicator in the middle of the reservoir - look inside for a protrusion from the centre stem with some small stamped writing on.