Before 1998 all m52b28's were single vanos (im almost positive).
The vanos system really isnt as complicated as its made out to be. It uses the rising oil pressure created while revving the engine higher in the rpm's to hydrolically force the camshaft to rotate (12.5 degrees) advancing the timing and therefore increasing horsepower and torque. The vanos system is activated by a solenoid controlled by the DME, when the oil pressure rises to a certain amount, vanos will kick in, thus explaining that jolt you get at about 3800 RPMs, you know what im talking about
. On the m50 and m52 only the intake camshaft is vanos controlled.
Its a bit more complicated than this, but thats the basic idea.
This write up explains it quite well. Although before i actually saw pictures of the system i couldnt grasp it all that clearly.
Quote:
OK.. here we go..
In normal overhead cam engines, the cams are connected to the crankshaft by either a belt or chain and some sort of gears..
In M50, M52, and S50 motors , we have chain and sprockets (like on a bike)..
The crankshaft drives a sprocket on the exhaust cam.. the exhaust cam sprocket is BOLTED to the exhaust cam... a second set of teeth on it are moving a second chain that goes "across" to the intake cam.. here's where the magic is..
The big sprocket on the intake cam is NOT bolted to the cam.. it's got a big hole in the middle of it.. in the inside of that hole is a helical set of teeth (helical means they curve to the left or right as they go thru the metal.. not straight)
Now.. on the end of the CAM is a little gear that is also helical on the OUTSIDE but it's too small to connect with the teeth on the inside of the big sprocket..
There is a little cup of metal.. with helical teeth to match the cam on the inside and to match the sprocket on the outside.... now.. as the sprocket turns.. it turns the cup thru one set of teeth, and the cup turns the little gear inside which, being bolted to the cam.. turns it.. so now.. the chain from the turning exhaust cam is also turning the intake cam.. although a bit indirectly 
The V (Variable) in Vanos is due to the HELICAL nature of those teeth.. if you push the cup in.. the relative sync of the cam to the sprocket is changed.. same if you pull it out some.. in our BMW's, the full travel of that gear will cause the cam to move a full 12.5 degrees while the outer sprocket doesn't move..
Now.. what moves this cup gear?? .. a hydraulic mechanism that works on oil pressure controlled by the DME..
At idle, the cam timing is retarded.. just off idle, the DME energizes a solenoid which allows oil pressure to move that cup gear to advance the cam 12.5 degrees at midrange, and then at about 5000rpm, it allows it to come back to the original position..
The greater advance causes better cylinder fill at mid rpms for better torque..
The noise some people complain about is usually tolerances that make the sprocket wiggle a bit as the cup gear is moved in or out.. sounds like a bunch of marbles in a box.. and it comes from the top DRIVERS side of the motor.. near the bump in the front of the cylinder head where the Vanos system is.
Hope this helps..
Jim Conforti
PS: And ALL Vanos motors make that marbly, rattly, "gargling" noise.. some owners just are a bit more sensitive to noises like that..
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Here is the vanos system installed in the engine.
Here is the vanos sprocket, not pushed into the double sided sprocket assembly located on the timing chaing sprocket.
Here is the vanos system removed from the car.
Again, here you see the vanos system removed from the engine, the vanos sprocket is pushed in and out to advance the timing, when it is out the timing is 'normal', or how it would be at idle, when it is pushed into the timing chain sprocket assembly, that is when it advances the timing.
Here you can see the timing chain vanos sprocket assembly, with the vanos system removed from the engine.
Here is a picture of the engine with the valve cover simply removed, incase you were having trouble grasping where the vanos assembly was located. (It is the semi-circle metal box located infront on the timing chains.)
Thanks to pelican parts for the pictures, sorry theyre so big though, hah. (Commentary is by me) I hope that explains it... its not too complicated after you begin to grasp the entire system.