Do you hear the noise primarily on acceleration? Does it get louder if you accelerate harder? Does it diminish or cease if you coast?

Rack & Pinion isn't going to tick-tick-tick when you're cornering and holding the steering wheel steady.

HOWEVER - CV joints make this EXACT sound when they're going out. Since your car is a 4MATIC, you've got 4 CV joints in the front connecting the transfer case to the front tires. The outer joints are the ones that usually fail due to the amount of flex they go through on a regular basis. Sometimes it's from a broken boot allowing all the grease to get out and the joint to run dry. You'd see one nasty mess behind the tire and perhaps on the inside of the rim if this were the case. CV joint grease is nasty beyond all belief. Makes axel grease look like cake icing.

Joints can still fail just due to plain wear and tear. They don't have to fail as a set - they can go out one-at-a-time.

Quick diagnostic - find a place in the yellow-pages that does R&R on CV joints. There are lots of places where I live that make that type of repair their sole business. They can quickly diagnose the click and confirm whether it's your CV or not.

When you find that it was the CV and get it fixed by someone other than the idiot at the dealership - take the repair order back to them and point out the error of their ways. OF COURSE, you still may need a rack & pinion - but if I'm right, the ticking noise is NOT the rack.