Here's the video of my car and the as-of-yet mystery knock. And before there's any hazing, the right fender liner was rattling around... you'll see...
http://www.youtube.com/watch?v=98E4JnTs6aM other updates:
* haven't drained the oil yet.
* apparently I don't have the oil filter cap remover tool.
* oil level is good, oil looks and smells good, no particulate on the dipstick.
* coolant level was low, but there was coolant present. (paging Gimp - when was it full?)
* All spark plugs looked pretty normal, and all were consistent.
* just a little milky junk under the oil fill cap after I ran the motor for a few minutes. Could be condensation mixing with the oil - it's cold here.
* I did smell some coolant in the cabin on the ride up from MD. Could be leaking from somewhere in the engine compartment, or could be the heater core?
Thanks for the help!