I purchased the MBZ STAR SERVICE DVD for Model 215 and 220. It is a fantastic resource and it addressed the above problem and many others common to the S/CL models.

My car had the 'drone' sound with vibration and was fixed by replacing the Pulsation Damper/Accumulator. My hydraulic lines were contacting the body, but the vibration remained, so the tech changed the Accumlator and problem solved.
I hope this helps.

Part was $200 cdn. Labour was 1 hr.

The MB STAR SERVICE Service Manual Library DVD states in the DTB section....

The ABC Pulsation Damper/Accumulator can correct a 'hum' or 'drone' sound.

If a 'hum' or 'drone' sound up to 2000 rpm is heard, but gets drowned out by the sound of the engine above 2000 rpm, it could be as simple as the hydraulic lines contacting the body of the car.

Have the lines inspected and if they clear the body, then replace the Accumulator.

The dvd manual also specifically states that replacing the ABC/PS pump will NOT correct the 'drone' sound problem.

I highly recommend buying the DVD from the dealer. It has all repair and maintenance procedures, diagnostic info, function descriptions, wiring diagrams with component locations. Service bulletins and DTB.
DVD price = $100 cdn.