Got it out. I found a local Mercedes mechanic that would come by the house and have a crack at it, off the record of course. He was able to heat it up with a torch and make a slit for a flathead. A few taps of a mallet and out it came. Even offered to buy the car off me if I ever became interested in selling.

Unfortunatly though the noise returned. He took another look and found it was the alternator. Which makes sense since it was a cheapo fix while out on a road trip no long ago. Nevertheless, he took a look at the old water pump and comfirmed the bearings were on their way out. Not failing yet, but it was definitely coming. So this turned into a preventative maintenance job as opposed to a fix. Fortunately the alternator is an easy swap.

For the forum search record, the service manual is wrong in instructing to tighten all water pump bolts to 35nm. Only the M8 bolts should be this tight, M6 will snap right off. If using the 2004 Service Manual, refer to ZH 7-14 for correct torque specs.