I didn't hear anything until I pulled into my driveway and put the car in park. Then the knocking sound was conspicuously present. My first thought: "Water pump is bad." I popped the hood and when I opened the driver's door to step out, I realized the knocking was coming from beneath the vehicle. LIke under the driver's area.

Hmm. Good news, not a water pump. Probably a busted exhaust hanger, since the rattle would go away at 1,200 rpm or higher; or when the transmission was engaged and foot on the brake.

If this were a truck, I'd crawl under and take a look. But trying to get under these W210s is like trying to look under the belly of a centipede. So I took it to my mechanic and we put it on the lift.

It took him about 40 seconds to locate the knock and then diagnose it.

Fractured catalytic converter -- the one off the driver-side exhaust manifold. This isn't going to be cheap with precious metals as expensive as they are.

Surprisingly, a call to his parts warehouse revealed a new Mercedes OEM pipe assembly and cat for under $900. Yeah, that's a lot of money, but it's about half what I thought it would be. Getting it installed next week. Let's hope the cat doesn't plug in the meantime.

Posting this for posterity to help in future diagnoses.