Well tonight after I came hope, I opened up the hood, and disassembled the fan shroud and front engine cover.
I had all my papers (diagrams from pfphipps, printout of this post) layed out on the garage floor... I couldn't find the SHOCK or tensioner or anything.
Luckily, I remembered there was a diagram of the Poly-V-Belt in my owner's manual... so I checked out the tensioner with the engine running, in D, with parking brake fully engaged. The tensioner didn't seem to be moving.
I then proceeded to look once again 2 inches over and 3 inches down from the thermostat housing... but just couldn't find the "shock" I was imagining. Well minutes later with the flashlight, I saw what appeared to be an actual SHOCK. DUH!
I then did Arthur's recommended test, put the car in D with parking brake on, used a lug nut wrench and pryed against the shock... NO RATTLING!!
I then checked to see if the belt was slipping or had slipped in the past just to be sure. There were 0 stored belt slips and 0 current slips (tested via Climate Control info). Good to go.
Unfortunately, I was unable to detect any movement with the shock... which means I must replace the shock? Now I only need to change the shock. I did see the top screw for the shock, I think I can handle that ok... but I could not see where the bottom of the shock went to... how would I access this? I'm thinking I might have to remove the belly pan?
I'm buying the part tomorrow from the dealer (but tonight I'll buy my door vacuum actuator from Mercedesshop, not as... 'urgent').
I'm very excited. This will be the first engine-related work (done a millionn interior things) I'll have done on the car (besides changing oil and filter)! I'm thrilled!!
I really appreciate everyone's help, especially pfphipps and arthur dalton, I couldn't have done it without you guys. THANKS AGAIN everyone!!
**2 hours later**
I just spoke with a friend of mine online who used to be a technician, and he said that shop time for changing the shock is 1 to 1 1/2 hours... that I would need a special tool and that I would need to remove the fan...... I'm not so excited anymore...
Basically, he recommended that I take it in.... but I could still try.
(A challenge would probably be good for me anyway......!)