Mercedes Repair Questions? Ask a Mechanic for Answers ASAP
The secondary air pump only exists to get the catalyst up to operating temp as quickly as possible. You'll get a check engine light, but no damage. When does the knocking occur, just when the engine is running, or only when car is moving, only in a certain gear, etc? Over bumps? Are you sure the knock and leak are not a damaged power steering rack? Take a look at the accordion boots to see if they are leaking fluid, in that case you need a new rack.
Youre going to need some way of getting underneath to see where the leak is coming from, and to feel for any looseness. Best if it were on a hoist, so the wheels would be hanging free and could be checked for suspension looseness.
I've worked for VW dealers for 15 years, but as a German car tech we work on a lot of different things, as they come in as used cars. I've even had the misfortune of working on American cars!