I got into the right front wheel-well today to route my fender repeater lights and to get a better look at the main crank area.

The first thing I saw when I peeled back the wheel well liner was a big chunk of loose rubber.

The "front" motor mount failed completely.

I levered up the engine (while idling) and almost all of the noises vanished. The engine has been vibrating directly against the sub-frame.

Not having a replacement motor mount handy, I cut a few short pieces of heater hose and slid them under the failed mount. The engine now idles smooth and easy.

NOTE: This is 100% temporary and the car won't be driven around like this!

In addition, I confirmed that my exhaust system is not rotten out or broken.
I replaced the serpentine belt, I also finished up the tune-up (OEM plugs, cap, rotor, wires, air filter).

Between all of it, I think I've tackled the whacky noises. MOST (but not all) of the noise was from the failed engine mount.

What's left? somewhat loud noises coming from the fuel injectors and some oil drip under the engine. The injectors on my c70 are pretty loud too, so there might now be much to be done there.

The oil drips are at least "quiet"

Thanks for all the advice, guys!